Method and apparatus for processing video signal

ABSTRACT

A method for decoding a video according to the present invention may comprise: deriving a plurality of reference sample lines for a current block, selecting at least one among the plurality of reference sample lines, determining whether to apply an intra filter to a reference sample included in the selected reference sample line, selectively applying the intra filter to the reference sample according to the determination, and performing intra prediction for the current block using the reference sample.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.16/908,197 (filed on Jun. 22, 2020), which is a Divisional of U.S.patent application Ser. No. 16/311,582 (filed on Dec. 19, 2018), nowissued as U.S. Pat. No. 10,735,720, which is a National Stage PatentApplication of PCT International Patent Application No.PCT/KR2017/006609 (filed on Jun. 22, 2017) under 35 U.S.C. § 371, whichclaims priority to Korean Patent Application Nos. 10-2016-0079641 (filedon Jun. 24, 2016), 10-2016-0079642 (filed on Jun. 24, 2016) and10-2016-0079644 (filed on Jun. 24, 2016), the teachings of which areincorporated herein in their entireties by reference.

TECHNICAL FIELD

The present invention relates to a method and an apparatus forprocessing video signal.

BACKGROUND ART

Recently, demands for high-resolution and high-quality images such ashigh definition (HD) images and ultra-high definition (UHD) images haveincreased in various application fields. However, higher resolution andquality image data has increasing amounts of data in comparison withconventional image data. Therefore, when transmitting image data byusing a medium such as conventional wired and wireless broadbandnetworks, or when storing image data by using a conventional storagemedium, costs of transmitting and storing increase. In order to solvethese problems occurring with an increase in resolution and quality ofimage data, high-efficiency image encoding/decoding techniques may beutilized.

Image compression technology includes various techniques, including: aninter-prediction technique of predicting a pixel value included in acurrent picture from a previous or subsequent picture of the currentpicture; an intra-prediction technique of predicting a pixel valueincluded in a current picture by using pixel information in the currentpicture; an entropy encoding technique of assigning a short code to avalue with a high appearance frequency and assigning a long code to avalue with a low appearance frequency; etc. Image data may beeffectively compressed by using such image compression technology, andmay be transmitted or stored.

In the meantime, with demands for high-resolution images, demands forstereographic image content, which is a new image service, have alsoincreased. A video compression technique for effectively providingstereographic image content with high resolution and ultra-highresolution is being discussed.

DISCLOSURE Technical Problem

An object of the present invention is intended to provide a method andan apparatus for efficiently performing intra-prediction for anencoding/decoding target block in encoding/decoding a video signal.

An object of the present invention is intended to provide a method andan apparatus for performing intra prediction for an encoding/decodingtarget block based on a plurality of reference lines.

An object of the present invention is intended to provide a method andan apparatus to apply an intra filter to at least one of a plurality ofreference lines.

An object of the present invention is intended to provide a method andan apparatus to adaptively determine an intra prediction mode or anumber of the intra prediction mode according to a reference line usedfor intra prediction of a current block.

The technical objects to be achieved by the present invention are notlimited to the above-mentioned technical problems. And, other technicalproblems that are not mentioned will be apparently understood to thoseskilled in the art from the following description.

Technical Solution

A method and an apparatus for decoding a video signal according to thepresent invention may derive a plurality of reference sample lines for acurrent block, select at least one among the plurality of referencesample lines, determine whether to apply an intra filter to a referencesample included in the selected reference sample line, selectively applythe intra filter to the reference sample according to the determination,and perform intra prediction for the current block using the referencesample.

A method and an apparatus for encoding a video signal according to thepresent invention may derive a plurality of reference sample lines for acurrent block, select at least one among the plurality of referencesample lines, determine whether to apply an intra filter to a referencesample included in the selected reference sample line, selectively applythe intra filter to the reference sample according to the determination,and perform intra prediction for the current block using the referencesample.

In the method and the apparatus for encoding/decoding a video signalaccording to the present invention, whether to apply the intra filter isdetermined based on a size of the current block, an intra predictionmode of the current block or a position of the selected referencesample.

In the method and the apparatus for encoding/decoding a video signalaccording to the present invention, a type of the intra filter isdetermined based on a size of the current block, an intra predictionmode of the current block or a position of the selected referencesample.

In the method and the apparatus for encoding/decoding a video signalaccording to the present invention, wherein the plurality of referencesample lines are classified into a plurality of groups, and a type ofthe intra filter is determined according to a group in which theselected reference sample line is included.

In the method and the apparatus for encoding/decoding a video signalaccording to the present invention, a number of intra prediction modesavailable for the current block is adaptively determined based on aposition of the selected reference sample line.

In the method and the apparatus for encoding/decoding a video signalaccording to the present invention, whether the current block isavailable to use a non-direction intra prediction mode is determinedbased on a position of the selected reference sample line.

In the method and the apparatus for encoding/decoding a video signalaccording to the present invention, selecting at least one among theplurality of reference sample lines is performed based on an intraprediction mode of the current block or a number of intra predictionmode available for the current block.

The features briefly summarized above for the present invention are onlyillustrative aspects of the detailed description of the invention thatfollows, but do not limit the scope of the invention.

Advantageous Effects

According to the present invention, an efficient intra-prediction may beperformed for an encoding/decoding target block.

According to the present invention, intra prediction for anencoding/decoding target block may be performed based on a plurality ofreference lines.

According to the present invention, an intra filter may be applied to atleast one of a plurality of reference lines.

According to the present invention, an intra prediction mode or a numberof the intra prediction mode may be adaptively determined according to areference line used for intra prediction of a current block.

The effects obtainable by the present invention are not limited to theabove-mentioned effects, and other effects not mentioned can be clearlyunderstood by those skilled in the art from the description below.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a device for encoding a videoaccording to an embodiment of the present invention.

FIG. 2 is a block diagram illustrating a device for decoding a videoaccording to an embodiment of the present invention.

FIG. 3 is a diagram illustrating an example of hierarchicallypartitioning a coding block based on a tree structure according to anembodiment of the present invention.

FIG. 4 is a diagram illustrating a partition type in which binarytree-based partitioning is allowed according to an embodiment of thepresent invention.

FIG. 5 is a diagram illustrating an example in which only a binarytree-based partition of a pre-determined type is allowed according to anembodiment of the present invention.

FIG. 6 is a diagram for explaining an example in which informationrelated to the allowable number of binary tree partitioning isencoded/decoded, according to an embodiment to which the presentinvention is applied.

FIG. 7 is a diagram illustrating a partition mode applicable to a codingblock according to an embodiment of the present invention.

FIG. 8 is a diagram illustrating types of pre-defined intra predictionmodes for a device for encoding/decoding a video according to anembodiment of the present invention.

FIG. 9 is a diagram illustrating a kind of extended intra predictionmodes according to an embodiment of the present invention.

FIG. 10 is a flowchart briefly illustrating an intra prediction methodaccording to an embodiment of the present invention.

FIG. 11 is a diagram illustrating a method of correcting a predictionsample of a current block based on differential information ofneighboring samples according to an embodiment of the present invention.

FIGS. 12 and 13 are diagrams illustrating a method of correcting aprediction sample based on a predetermined correction filter accordingto an embodiment of the present invention.

FIG. 14 shows a range of reference samples for intra predictionaccording to an embodiment to which the present invention is applied.

FIGS. 15 to 17 illustrate an example of filtering on reference samplesaccording to an embodiment of the present invention.

FIG. 18 is a diagram illustrating a plurality of reference sample linesaccording to an embodiment of the present invention.

FIGS. 19A and 19B are diagrams illustrating an example in which a use ofan extended reference line is determined according to a shape of acurrent block, according to an embodiment of the present invention.

FIG. 20 is a flowchart illustrating a method of performingintra-prediction using an extended reference line according to thepresent invention.

FIG. 21 is a diagram illustrating a plurality of reference lines for anon-square block according to the present invention.

FIG. 22 is a diagram for explaining an example in which an unavailablereference sample is replaced with an available reference sample locatedat the shortest distance from the unavailable reference sample.

FIGS. 23 and 24 are diagrams for explaining an embodiment in which theposition of an available reference sample is determined adaptivelyaccording to a distance between an unavailable reference sample and anavailable reference sample included in the same reference line as theunavailable reference sample.

FIGS. 25 and 26 are diagrams illustrating reference samples used toderive an average value of a reference line according to an embodimentto which the present invention is applied.

FIG. 27 is a flowchart illustrating processes of obtaining a residualsample according to an embodiment to which the present invention isapplied.

MODE FOR INVENTION

A variety of modifications may be made to the present invention andthere are various embodiments of the present invention, examples ofwhich will now be provided with reference to drawings and described indetail. However, the present invention is not limited thereto, and theexemplary embodiments can be construed as including all modifications,equivalents, or substitutes in a technical concept and a technical scopeof the present invention. The similar reference numerals refer to thesimilar element in described the drawings.

Terms used in the specification, ‘first’, ‘second’, etc. can be used todescribe various components, but the components are not to be construedas being limited to the terms. The terms are only used to differentiateone component from other components. For example, the ‘first’ componentmay be named the ‘second’ component without departing from the scope ofthe present invention, and the ‘second’ component may also be similarlynamed the ‘first’ component. The term ‘and/or’ includes a combination ofa plurality of items or any one of a plurality of terms.

It will be understood that when an element is simply referred to asbeing ‘connected to’ or ‘coupled to’ another element without being‘directly connected to’ or ‘directly coupled to’ another element in thepresent description, it may be ‘directly connected to’ or ‘directlycoupled to’ another element or be connected to or coupled to anotherelement, having the other element intervening therebetween. In contrast,it should be understood that when an element is referred to as being“directly coupled” or “directly connected” to another element, there areno intervening elements present.

The terms used in the present specification are merely used to describeparticular embodiments, and are not intended to limit the presentinvention. An expression used in the singular encompasses the expressionof the plural, unless it has a clearly different meaning in the context.In the present specification, it is to be understood that terms such as“including”, “having”, etc. are intended to indicate the existence ofthe features, numbers, steps, actions, elements, parts, or combinationsthereof disclosed in the specification, and are not intended to precludethe possibility that one or more other features, numbers, steps,actions, elements, parts, or combinations thereof may exist or may beadded.

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.Hereinafter, the same constituent elements in the drawings are denotedby the same reference numerals, and a repeated description of the sameelements will be omitted.

FIG. 1 is a block diagram illustrating a device for encoding a videoaccording to an embodiment of the present invention.

Referring to FIG. 1, the device 100 for encoding a video may include: apicture partitioning module 110, prediction modules 120 and 125, atransform module 130, a quantization module 135, a rearrangement module160, an entropy encoding module 165, an inverse quantization module 140,an inverse transform module 145, a filter module 150, and a memory 155.

The constitutional parts shown in FIG. 1 are independently shown so asto represent characteristic functions different from each other in thedevice for encoding a video. Thus, it does not mean that eachconstitutional part is constituted in a constitutional unit of separatedhardware or software. In other words, each constitutional part includeseach of enumerated constitutional parts for convenience. Thus, at leasttwo constitutional parts of each constitutional part may be combined toform one constitutional part or one constitutional part may be dividedinto a plurality of constitutional parts to perform each function. Theembodiment where each constitutional part is combined and the embodimentwhere one constitutional part is divided are also included in the scopeof the present invention, if not departing from the essence of thepresent invention.

Also, some of constituents may not be indispensable constituentsperforming essential functions of the present invention but be selectiveconstituents improving only performance thereof. The present inventionmay be implemented by including only the indispensable constitutionalparts for implementing the essence of the present invention except theconstituents used in improving performance. The structure including onlythe indispensable constituents except the selective constituents used inimproving only performance is also included in the scope of the presentinvention.

The picture partitioning module 110 may partition an input picture intoone or more processing units. Here, the processing unit may be aprediction unit (PU), a transform unit (TU), or a coding unit (CU). Thepicture partitioning module 110 may partition one picture intocombinations of multiple coding units, prediction units, and transformunits, and may encode a picture by selecting one combination of codingunits, prediction units, and transform units with a predeterminedcriterion (e.g., cost function).

For example, one picture may be partitioned into multiple coding units.A recursive tree structure, such as a quad tree structure, may be usedto partition a picture into coding units. A coding unit which ispartitioned into other coding units with one picture or a largest codingunit as a root may be partitioned with child nodes corresponding to thenumber of partitioned coding units. A coding unit which is no longerpartitioned by a predetermined limitation serves as a leaf node. Thatis, when it is assumed that only square partitioning is possible for onecoding unit, one coding unit may be partitioned into four other codingunits at most.

Hereinafter, in the embodiment of the present invention, the coding unitmay mean a unit performing encoding, or a unit performing decoding.

A prediction unit may be one of partitions partitioned into a square ora rectangular shape having the same size in a single coding unit, or aprediction unit may be one of partitions partitioned so as to have adifferent shape/size in a single coding unit.

When a prediction unit subjected to intra prediction is generated basedon a coding unit and the coding unit is not the smallest coding unit,intra prediction may be performed without partitioning the coding unitinto multiple prediction units N×N.

The prediction modules 120 and 125 may include an inter predictionmodule 120 performing inter prediction and an intra prediction module125 performing intra prediction. Whether to perform inter prediction orintra prediction for the prediction unit may be determined, and detailedinformation (e.g., an intra prediction mode, a motion vector, areference picture, etc.) according to each prediction method may bedetermined. Here, the processing unit subjected to prediction may bedifferent from the processing unit for which the prediction method anddetailed content is determined. For example, the prediction method, theprediction mode, etc. may be determined by the prediction unit, andprediction may be performed by the transform unit. A residual value(residual block) between the generated prediction block and an originalblock may be input to the transform module 130. Also, prediction modeinformation, motion vector information, etc. used for prediction may beencoded with the residual value by the entropy encoding module 165 andmay be transmitted to a device for decoding a video. When a particularencoding mode is used, it is possible to transmit to a device fordecoding video by encoding the original block as it is withoutgenerating the prediction block through the prediction modules 120 and125.

The inter prediction module 120 may predict the prediction unit based oninformation of at least one of a previous picture or a subsequentpicture of the current picture, or may predict the prediction unit basedon information of some encoded regions in the current picture, in somecases. The inter prediction module 120 may include a reference pictureinterpolation module, a motion prediction module, and a motioncompensation module.

The reference picture interpolation module may receive reference pictureinformation from the memory 155 and may generate pixel information of aninteger pixel or less then the integer pixel from the reference picture.In the case of luma pixels, an 8-tap DCT-based interpolation filterhaving different filter coefficients may be used to generate pixelinformation of an integer pixel or less than an integer pixel in unitsof a ¼ pixel. In the case of chroma signals, a 4-tap DCT-basedinterpolation filter having different filter coefficient may be used togenerate pixel information of an integer pixel or less than an integerpixel in units of a ⅛ pixel.

The motion prediction module may perform motion prediction based on thereference picture interpolated by the reference picture interpolationmodule. As methods for calculating a motion vector, various methods,such as a full search-based block matching algorithm (FBMA), a threestep search (TSS), a new three-step search algorithm (NTS), etc., may beused. The motion vector may have a motion vector value in units of a ½pixel or a ¼ pixel based on an interpolated pixel. The motion predictionmodule may predict a current prediction unit by changing the motionprediction method. As motion prediction methods, various methods, suchas a skip method, a merge method, an AMVP (Advanced Motion VectorPrediction) method, an intra block copy method, etc., may be used.

The intra prediction module 125 may generate a prediction unit based onreference pixel information neighboring to a current block which ispixel information in the current picture. When the neighboring block ofthe current prediction unit is a block subjected to inter prediction andthus a reference pixel is a pixel subjected to inter prediction, thereference pixel included in the block subjected to inter prediction maybe replaced with reference pixel information of a neighboring blocksubjected to intra prediction. That is, when a reference pixel is notavailable, at least one reference pixel of available reference pixelsmay be used instead of unavailable reference pixel information.

Prediction modes in intra prediction may include a directionalprediction mode using reference pixel information depending on aprediction direction and a non-directional prediction mode not usingdirectional information in performing prediction. A mode for predictingluma information may be different from a mode for predicting chromainformation, and in order to predict the chroma information, intraprediction mode information used to predict luma information orpredicted luma signal information may be utilized.

In performing intra prediction, when the size of the prediction unit isthe same as the size of the transform unit, intra prediction may beperformed on the prediction unit based on pixels positioned at the left,the top left, and the top of the prediction unit. However, in performingintra prediction, when the size of the prediction unit is different fromthe size of the transform unit, intra prediction may be performed usinga reference pixel based on the transform unit. Also, intra predictionusing N×N partitioning may be used for only the smallest coding unit.

In the intra prediction method, a prediction block may be generatedafter applying an AIS (Adaptive Intra Smoothing) filter to a referencepixel depending on the prediction modes. The type of the AIS filterapplied to the reference pixel may vary. In order to perform the intraprediction method, an intra prediction mode of the current predictionunit may be predicted from the intra prediction mode of the predictionunit neighboring to the current prediction unit. In prediction of theprediction mode of the current prediction unit by using mode informationpredicted from the neighboring prediction unit, when the intraprediction mode of the current prediction unit is the same as the intraprediction mode of the neighboring prediction unit, informationindicating that the prediction modes of the current prediction unit andthe neighboring prediction unit are equal to each other may betransmitted using predetermined flag information. When the predictionmode of the current prediction unit is different from the predictionmode of the neighboring prediction unit, entropy encoding may beperformed to encode prediction mode information of the current block.

Also, a residual block including information on a residual value whichis a different between the prediction unit subjected to prediction andthe original block of the prediction unit may be generated based onprediction units generated by the prediction modules 120 and 125. Thegenerated residual block may be input to the transform module 130.

The transform module 130 may transform the residual block including theinformation on the residual value between the original block and theprediction unit generated by the prediction modules 120 and 125 by usinga transform method, such as discrete cosine transform (DCT), discretesine transform (DST), and KLT. Whether to apply DCT, DST, or KLT inorder to transform the residual block may be determined based on intraprediction mode information of the prediction unit used to generate theresidual block.

The quantization module 135 may quantize values transformed to afrequency domain by the transform module 130. Quantization coefficientsmay vary depending on the block or importance of a picture. The valuescalculated by the quantization module 135 may be provided to the inversequantization module 140 and the rearrangement module 160.

The rearrangement module 160 may rearrange coefficients of quantizedresidual values.

The rearrangement module 160 may change a coefficient in the form of atwo-dimensional block into a coefficient in the form of aone-dimensional vector through a coefficient scanning method. Forexample, the rearrangement module 160 may scan from a DC coefficient toa coefficient in a high frequency domain using a zigzag scanning methodso as to change the coefficients to be in the form of one-dimensionalvectors. Depending on the size of the transform unit and the intraprediction mode, vertical direction scanning where coefficients in theform of two-dimensional blocks are scanned in the column direction orhorizontal direction scanning where coefficients in the form oftwo-dimensional blocks are scanned in the row direction may be usedinstead of zigzag scanning. That is, which scanning method among zigzagscanning, vertical direction scanning, and horizontal direction scanningis used may be determined depending on the size of the transform unitand the intra prediction mode.

The entropy encoding module 165 may perform entropy encoding based onthe values calculated by the rearrangement module 160. Entropy encodingmay use various encoding methods, for example, exponential Golombcoding, context-adaptive variable length coding (CAVLC), andcontext-adaptive binary arithmetic coding (CABAC).

The entropy encoding module 165 may encode a variety of information,such as residual value coefficient information and block typeinformation of the coding unit, prediction mode information, partitionunit information, prediction unit information, transform unitinformation, motion vector information, reference frame information,block interpolation information, filtering information, etc. from therearrangement module 160 and the prediction modules 120 and 125.

The entropy encoding module 165 may entropy encode the coefficients ofthe coding unit input from the rearrangement module 160.

The inverse quantization module 140 may inversely quantize the valuesquantized by the quantization module 135 and the inverse transformmodule 145 may inversely transform the values transformed by thetransform module 130. The residual value generated by the inversequantization module 140 and the inverse transform module 145 may becombined with the prediction unit predicted by a motion estimationmodule, a motion compensation module, and the intra prediction module ofthe prediction modules 120 and 125 such that a reconstructed block canbe generated.

The filter module 150 may include at least one of a deblocking filter,an offset correction unit, and an adaptive loop filter (ALF).

The deblocking filter may remove block distortion that occurs due toboundaries between the blocks in the reconstructed picture. In order todetermine whether to perform deblocking, the pixels included in severalrows or columns in the block may be a basis of determining whether toapply the deblocking filter to the current block. When the deblockingfilter is applied to the block, a strong filter or a weak filter may beapplied depending on required deblocking filtering strength. Also, inapplying the deblocking filter, horizontal direction filtering andvertical direction filtering may be processed in parallel.

The offset correction module may correct offset with the originalpicture in units of a pixel in the picture subjected to deblocking. Inorder to perform the offset correction on a particular picture, it ispossible to use a method of applying offset in consideration of edgeinformation of each pixel or a method of partitioning pixels of apicture into the predetermined number of regions, determining a regionto be subjected to perform offset, and applying the offset to thedetermined region.

Adaptive loop filtering (ALF) may be performed based on the valueobtained by comparing the filtered reconstructed picture and theoriginal picture. The pixels included in the picture may be divided intopredetermined groups, a filter to be applied to each of the groups maybe determined, and filtering may be individually performed for eachgroup. Information on whether to apply ALF and a luma signal may betransmitted by coding units (CU). The shape and filter coefficient of afilter for ALF may vary depending on each block. Also, the filter forALF in the same shape (fixed shape) may be applied regardless ofcharacteristics of the application target block.

The memory 155 may store the reconstructed block or picture calculatedthrough the filter module 150. The stored reconstructed block or picturemay be provided to the prediction modules 120 and 125 in performinginter prediction.

FIG. 2 is a block diagram illustrating a device for decoding a videoaccording to an embodiment of the present invention.

Referring to FIG. 2, the device 200 for decoding a video may include: anentropy decoding module 210, a rearrangement module 215, an inversequantization module 220, an inverse transform module 225, predictionmodules 230 and 235, a filter module 240, and a memory 245.

When a video bitstream is input from the device for encoding a video,the input bitstream may be decoded according to an inverse process ofthe device for encoding a video.

The entropy decoding module 210 may perform entropy decoding accordingto an inverse process of entropy encoding by the entropy encoding moduleof the device for encoding a video. For example, corresponding to themethods performed by the device for encoding a video, various methods,such as exponential Golomb coding, context-adaptive variable lengthcoding (CAVLC), and context-adaptive binary arithmetic coding (CABAC)may be applied.

The entropy decoding module 210 may decode information on intraprediction and inter prediction performed by the device for encoding avideo.

The rearrangement module 215 may perform rearrangement on the bitstreamentropy decoded by the entropy decoding module 210 based on therearrangement method used in the device for encoding a video. Therearrangement module may reconstruct and rearrange the coefficients inthe form of one-dimensional vectors to the coefficient in the form oftwo-dimensional blocks. The rearrangement module 215 may receiveinformation related to coefficient scanning performed in the device forencoding a video and may perform rearrangement via a method of inverselyscanning the coefficients based on the scanning order performed in thedevice for encoding a video.

The inverse quantization module 220 may perform inverse quantizationbased on a quantization parameter received from the device for encodinga video and the rearranged coefficients of the block.

The inverse transform module 225 may perform the inverse transform,i.e., inverse DCT, inverse DST, and inverse KLT, which is the inverseprocess of transform, i.e., DCT, DST, and KLT, performed by thetransform module on the quantization result by the device for encoding avideo. Inverse transform may be performed based on a transfer unitdetermined by the device for encoding a video. The inverse transformmodule 225 of the device for decoding a video may selectively performtransform schemes (e.g., DCT, DST, and KLT) depending on multiple piecesof information, such as the prediction method, the size of the currentblock, the prediction direction, etc.

The prediction modules 230 and 235 may generate a prediction block basedon information on prediction block generation received from the entropydecoding module 210 and previously decoded block or picture informationreceived from the memory 245.

As described above, like the operation of the device for encoding avideo, in performing intra prediction, when the size of the predictionunit is the same as the size of the transform unit, intra prediction maybe performed on the prediction unit based on the pixels positioned atthe left, the top left, and the top of the prediction unit. Inperforming intra prediction, when the size of the prediction unit isdifferent from the size of the transform unit, intra prediction may beperformed using a reference pixel based on the transform unit. Also,intra prediction using N×N partitioning may be used for only thesmallest coding unit.

The prediction modules 230 and 235 may include a prediction unitdetermination module, an inter prediction module, and an intraprediction module. The prediction unit determination module may receivea variety of information, such as prediction unit information,prediction mode information of an intra prediction method, informationon motion prediction of an inter prediction method, etc. from theentropy decoding module 210, may divide a current coding unit intoprediction units, and may determine whether inter prediction or intraprediction is performed on the prediction unit. By using informationrequired in inter prediction of the current prediction unit receivedfrom the device for encoding a video, the inter prediction module 230may perform inter prediction on the current prediction unit based oninformation of at least one of a previous picture or a subsequentpicture of the current picture including the current prediction unit.Alternatively, inter prediction may be performed based on information ofsome pre-reconstructed regions in the current picture including thecurrent prediction unit.

In order to perform inter prediction, it may be determined for thecoding unit which of a skip mode, a merge mode, an AMVP mode, and aninter block copy mode is used as the motion prediction method of theprediction unit included in the coding unit.

The intra prediction module 235 may generate a prediction block based onpixel information in the current picture. When the prediction unit is aprediction unit subjected to intra prediction, intra prediction may beperformed based on intra prediction mode information of the predictionunit received from the device for encoding a video. The intra predictionmodule 235 may include an adaptive intra smoothing (AIS) filter, areference pixel interpolation module, and a DC filter. The AIS filterperforms filtering on the reference pixel of the current block, andwhether to apply the filter may be determined depending on theprediction mode of the current prediction unit. AIS filtering may beperformed on the reference pixel of the current block by using theprediction mode of the prediction unit and AIS filter informationreceived from the device for encoding a video. When the prediction modeof the current block is a mode where AIS filtering is not performed, theAIS filter may not be applied.

When the prediction mode of the prediction unit is a prediction mode inwhich intra prediction is performed based on the pixel value obtained byinterpolating the reference pixel, the reference pixel interpolationmodule may interpolate the reference pixel to generate the referencepixel of an integer pixel or less than an integer pixel. When theprediction mode of the current prediction unit is a prediction mode inwhich a prediction block is generated without interpolation thereference pixel, the reference pixel may not be interpolated. The DCfilter may generate a prediction block through filtering when theprediction mode of the current block is a DC mode.

The reconstructed block or picture may be provided to the filter module240. The filter module 240 may include the deblocking filter, the offsetcorrection module, and the ALF.

Information on whether or not the deblocking filter is applied to thecorresponding block or picture and information on which of a strongfilter and a weak filter is applied when the deblocking filter isapplied may be received from the device for encoding a video. Thedeblocking filter of the device for decoding a video may receiveinformation on the deblocking filter from the device for encoding avideo, and may perform deblocking filtering on the corresponding block.

The offset correction module may perform offset correction on thereconstructed picture based on the type of offset correction and offsetvalue information applied to a picture in performing encoding.

The ALF may be applied to the coding unit based on information onwhether to apply the ALF, ALF coefficient information, etc. receivedfrom the device for encoding a video. The ALF information may beprovided as being included in a particular parameter set.

The memory 245 may store the reconstructed picture or block for use as areference picture or block, and may provide the reconstructed picture toan output module.

As described above, in the embodiment of the present invention, forconvenience of explanation, the coding unit is used as a termrepresenting a unit for encoding, but the coding unit may serve as aunit performing decoding as well as encoding.

In addition, a current block may represent a target block to beencoded/decoded. And, the current block may represent a coding treeblock (or a coding tree unit), a coding block (or a coding unit), atransform block (or a transform unit), a prediction block (or aprediction unit), or the like depending on an encoding/decoding step.

A picture may be encoded/decoded by divided into base blocks having asquare shape or a non-square shape. At this time, the base block may bereferred to as a coding tree unit. The coding tree unit may be definedas a coding unit of the largest size allowed within a sequence or aslice. Information regarding whether the coding tree unit has a squareshape or has a non-square shape or information regarding a size of thecoding tree unit may be signaled through a sequence parameter set, apicture parameter set, or a slice header. The coding tree unit may bedivided into smaller size partitions. At this time, if it is assumedthat a depth of a partition generated by dividing the coding tree unitis 1, a depth of a partition generated by dividing the partition havingdepth 1 may be defined as 2. That is, a partition generated by dividinga partition having a depth k in the coding tree unit may be defined ashaving a depth k+1.

A partition of arbitrary size generated by dividing a coding tree unitmay be defined as a coding unit. The coding unit may be recursivelydivided or divided into base units for performing prediction,quantization, transform, or in-loop filtering, and the like. Forexample, a partition of arbitrary size generated by dividing the codingunit may be defined as a coding unit, or may be defined as a transformunit or a prediction unit, which is a base unit for performingprediction, quantization, transform or in-loop filtering and the like.

Partitioning of a coding tree unit or a coding unit may be performedbased on at least one of a vertical line and a horizontal line. Inaddition, the number of vertical lines or horizontal lines partitioningthe coding tree unit or the coding unit may be at least one or more. Forexample, the coding tree unit or the coding unit may be divided into twopartitions using one vertical line or one horizontal line, or the codingtree unit or the coding unit may be divided into three partitions usingtwo vertical lines or two horizontal lines. Alternatively, the codingtree unit or the coding unit may be partitioned into four partitionshaving a height and a width of ½ by using one vertical line and onehorizontal line.

When a coding tree unit or a coding unit is divided into a plurality ofpartitions using at least one vertical line or at least one horizontalline, the partitions may have a uniform size or a different size.Alternatively, any one partition may have a different size from theremaining partitions.

In the embodiments described below, it is assumed that a coding treeunit or a coding unit is divided into a quad tree structure or a binarytree structure. However, it is also possible to divide a coding treeunit or a coding unit using a larger number of vertical lines or alarger number of horizontal lines.

FIG. 3 is a diagram illustrating an example of hierarchicallypartitioning a coding block based on a tree structure according to anembodiment of the present invention.

An input video signal is decoded in predetermined block units. Such adefault unit for decoding the input video signal is a coding block. Thecoding block may be a unit performing intra/inter prediction, transform,and quantization. In addition, a prediction mode (e.g., intra predictionmode or inter prediction mode) is determined in units of a coding block,and the prediction blocks included in the coding block may share thedetermined prediction mode. The coding block may be a square ornon-square block having an arbitrary size in a range of 8×8 to 64×64, ormay be a square or non-square block having a size of 128×128, 256×256,or more.

Specifically, the coding block may be hierarchically partitioned basedon at least one of a quad tree and a binary tree. Here, quad tree-basedpartitioning may mean that a 2N×2N coding block is partitioned into fourN×N coding blocks, and binary tree-based partitioning may mean that onecoding block is partitioned into two coding blocks. Even if the binarytree-based partitioning is performed, a square-shaped coding block mayexist in the lower depth.

Binary tree-based partitioning may be symmetrically or asymmetricallyperformed. The coding block partitioned based on the binary tree may bea square block or a non-square block, such as a rectangular shape. Forexample, a partition type in which the binary tree-based partitioning isallowed may comprise at least one of a symmetric type of 2N×N(horizontal directional non-square coding unit) or N×2N (verticaldirection non-square coding unit), asymmetric type of nL×2N, nR×2N,2N×nU, or 2N×nD.

Binary tree-based partitioning may be limitedly allowed to one of asymmetric or an asymmetric type partition. In this case, constructingthe coding tree unit with square blocks may correspond to quad tree CUpartitioning, and constructing the coding tree unit with symmetricnon-square blocks may correspond to binary tree partitioning.Constructing the coding tree unit with square blocks and symmetricnon-square blocks may correspond to quad and binary tree CUpartitioning.

Binary tree-based partitioning may be performed on a coding block wherequad tree-based partitioning is no longer performed. Quad tree-basedpartitioning may no longer be performed on the coding block partitionedbased on the binary tree.

Furthermore, partitioning of a lower depth may be determined dependingon a partition type of an upper depth. For example, if binary tree-basedpartitioning is allowed in two or more depths, only the same type as thebinary tree partitioning of the upper depth may be allowed in the lowerdepth. For example, if the binary tree-based partitioning in the upperdepth is performed with 2N×N type, the binary tree-based partitioning inthe lower depth is also performed with 2N×N type. Alternatively, if thebinary tree-based partitioning in the upper depth is performed with N×2Ntype, the binary tree-based partitioning in the lower depth is alsoperformed with N×2N type.

On the contrary, it is also possible to allow, in a lower depth, only atype different from a binary tree partitioning type of a upper depth.

It may be possible to limit only a specific type of binary tree basedpartitioning to be used for sequence, slice, coding tree unit, or codingunit. As an example, only 2N×N type or N×2N type of binary tree-basedpartitioning may be allowed for the coding tree unit. An availablepartition type may be predefined in an encoder or a decoder. Orinformation on available partition type or on unavailable partition typeon may be encoded and then signaled through a bitstream.

FIG. 5 is a diagram illustrating an example in which only a specifictype of binary tree-based partitioning is allowed. FIG. 5A shows anexample in which only N×2N type of binary tree-based partitioning isallowed, and FIG. 5B shows an example in which only 2N×N type of binarytree-based partitioning is allowed. In order to implement adaptivepartitioning based on the quad tree or binary tree, informationindicating quad tree-based partitioning, information on the size/depthof the coding block that quad tree-based partitioning is allowed,information indicating binary tree-based partitioning, information onthe size/depth of the coding block that binary tree-based partitioningis allowed, information on the size/depth of the coding block thatbinary tree-based partitioning is not allowed, information on whetherbinary tree-based partitioning is performed in a vertical direction or ahorizontal direction, etc. may be used.

In addition, information on the number of times a binary treepartitioning is allowed, a depth at which the binary tree partitioningis allowed, or the number of the depths at which the binary treepartitioning is allowed may be obtained for a coding tree unit or aspecific coding unit. The information may be encoded in units of acoding tree unit or a coding unit, and may be transmitted to a decoderthrough a bitstream.

For example, a syntax ‘max_binary_depth_idx_minus1’ indicating a maximumdepth at which binary tree partitioning is allowed may beencoded/decoded through a bitstream. In this case,max_binary_depth_idx_minus1+1 may indicate the maximum depth at whichthe binary tree partitioning is allowed.

Referring to the example shown in FIG. 6, in FIG. 6, the binary treepartitioning has been performed for a coding unit having a depth of 2and a coding unit having a depth of 3. Accordingly, at least one ofinformation indicating the number of times the binary tree partitioningin the coding tree unit has been performed (i.e., 2 times), informationindicating the maximum depth which the binary tree partitioning has beenallowed in the coding tree unit (i.e., depth 3), or the number of depthsin which the binary tree partitioning has been performed in the codingtree unit (i.e., 2 (depth 2 and depth 3)) may be encoded/decoded througha bitstream.

As another example, at least one of information on the number of timesthe binary tree partitioning is permitted, the depth at which the binarytree partitioning is allowed, or the number of the depths at which thebinary tree partitioning is allowed may be obtained for each sequence oreach slice. For example, the information may be encoded in units of asequence, a picture, or a slice unit and transmitted through abitstream. Accordingly, at least one of the number of the binary treepartitioning in a first slice, the maximum depth in which the binarytree partitioning is allowed in the first slice, or the number of depthsin which the binary tree partitioning is performed in the first slicemay be difference from a second slice. For example, in the first slice,binary tree partitioning may be permitted for only one depth, while inthe second slice, binary tree partitioning may be permitted for twodepths.

As another example, the number of times the binary tree partitioning ispermitted, the depth at which the binary tree partitioning is allowed,or the number of depths at which the binary tree partitioning is allowedmay be set differently according to a time level identifier (TemporalID)of a slice or a picture. Here, the temporal level identifier(TemporalID) is used to identify each of a plurality of layers of videohaving a scalability of at least one of view, spatial, temporal orquality.

As shown in FIG. 3, the first coding block 300 with the partition depth(split depth) of k may be partitioned into multiple second coding blocksbased on the quad tree. For example, the second coding blocks 310 to 340may be square blocks having the half width and the half height of thefirst coding block, and the partition depth of the second coding blockmay be increased to k+1.

The second coding block 310 with the partition depth of k+1 may bepartitioned into multiple third coding blocks with the partition depthof k+2. Partitioning of the second coding block 310 may be performed byselectively using one of the quad tree and the binary tree depending ona partitioning method. Here, the partitioning method may be determinedbased on at least one of the information indicating quad tree-basedpartitioning and the information indicating binary tree-basedpartitioning.

When the second coding block 310 is partitioned based on the quad tree,the second coding block 310 may be partitioned into four third codingblocks 310 a having the half width and the half height of the secondcoding block, and the partition depth of the third coding block 310 amay be increased to k+2. In contrast, when the second coding block 310is partitioned based on the binary tree, the second coding block 310 maybe partitioned into two third coding blocks. Here, each of two thirdcoding blocks may be a non-square block having one of the half width andthe half height of the second coding block, and the partition depth maybe increased to k+2. The second coding block may be determined as anon-square block of a horizontal direction or a vertical directiondepending on a partitioning direction, and the partitioning directionmay be determined based on the information on whether binary tree-basedpartitioning is performed in a vertical direction or a horizontaldirection.

In the meantime, the second coding block 310 may be determined as a leafcoding block that is no longer partitioned based on the quad tree or thebinary tree. In this case, the leaf coding block may be used as aprediction block or a transform block.

Like partitioning of the second coding block 310, the third coding block310 a may be determined as a leaf coding block, or may be furtherpartitioned based on the quad tree or the binary tree.

In the meantime, the third coding block 310 b partitioned based on thebinary tree may be further partitioned into coding blocks 310 b-2 of avertical direction or coding blocks 310 b-3 of a horizontal directionbased on the binary tree, and the partition depth of the relevant codingblocks may be increased to k+3. Alternatively, the third coding block310 b may be determined as a leaf coding block 310 b-1 that is no longerpartitioned based on the binary tree. In this case, the coding block 310b-1 may be used as a prediction block or a transform block. However, theabove partitioning process may be limitedly performed based on at leastone of the information on the size/depth of the coding block that quadtree-based partitioning is allowed, the information on the size/depth ofthe coding block that binary tree-based partitioning is allowed, and theinformation on the size/depth of the coding block that binary tree-basedpartitioning is not allowed.

A number of a candidate that represent a size of a coding block may belimited to a predetermined number, or a size of a coding block in apredetermined unit may have a fixed value. As an example, the size ofthe coding block in a sequence or in a picture may be limited to have256×256, 128×128, or 32×32. Information indicating the size of thecoding block in the sequence or in the picture may be signaled through asequence header or a picture header.

As a result of partitioning based on a quad tree and a binary tree, acoding unit may be represented as square or rectangular shape of anarbitrary size.

A coding block is encoded using at least one of a skip mode, intraprediction, inter prediction, or a skip method. Once a coding block isdetermined, a prediction block may be determined through predictivepartitioning of the coding block. The predictive partitioning of thecoding block may be performed by a partition mode (Part_mode) indicatinga partition type of the coding block. A size or a shape of theprediction block may be determined according to the partition mode ofthe coding block. For example, a size of a prediction block determinedaccording to the partition mode may be equal to or smaller than a sizeof a coding block.

FIG. 7 is a diagram illustrating a partition mode that may be applied toa coding block when the coding block is encoded by inter prediction.

When a coding block is encoded by inter prediction, one of 8partitioning modes may be applied to the coding block, as in the exampleshown in FIG. 4.

When a coding block is encoded by intra prediction, a partition modePART_2N×2N or a partition mode PART_N×N may be applied to the codingblock.

PART_N×N may be applied when a coding block has a minimum size. Here,the minimum size of the coding block may be pre-defined in an encoderand a decoder. Or, information regarding the minimum size of the codingblock may be signaled via a bitstream. For example, the minimum size ofthe coding block may be signaled through a slice header, so that theminimum size of the coding block may be defined per slice.

In general, a prediction block may have a size from 64×64 to 4×4.However, when a coding block is encoded by inter prediction, it may berestricted that the prediction block does not have a 4×4 size in orderto reduce memory bandwidth when performing motion compensation.

FIG. 8 is a diagram illustrating types of pre-defined intra predictionmodes for a device for encoding/decoding a video according to anembodiment of the present invention.

The device for encoding/decoding a video may perform intra predictionusing one of pre-defined intra prediction modes. The pre-defined intraprediction modes for intra prediction may include non-directionalprediction modes (e.g., a planar mode, a DC mode) and 33 directionalprediction modes.

Alternatively, in order to enhance accuracy of intra prediction, alarger number of directional prediction modes than the 33 directionalprediction modes may be used. That is, M extended directional predictionmodes may be defined by subdividing angles of the directional predictionmodes (M>33), and a directional prediction mode having a predeterminedangle may be derived using at least one of the 33 pre-defineddirectional prediction modes.

A larger number of intra prediction modes than 35 intra prediction modesshown in FIG. 8 may be used. For example, a larger number of intraprediction modes than the 35 intra prediction modes can be used bysubdividing angles of directional prediction modes or by deriving adirectional prediction mode having a predetermined angle using at leastone of a pre-defined number of directional prediction modes. At thistime, the use of a larger number of intra prediction modes than the 35intra prediction modes may be referred to as an extended intraprediction mode.

FIG. 9 shows an example of extended intra prediction modes, and theextended intra prediction modes may include two non-directionalprediction modes and 65 extended directional prediction modes. The samenumbers of the extended intra prediction modes may be used for a lumacomponent and a chroma component, or a different number of intraprediction modes may be used for each component. For example, 67extended intra prediction modes may be used for the luma component, and35 intra prediction modes may be used for the chroma component.

Alternatively, depending on the chroma format, a different number ofintra prediction modes may be used in performing intra prediction. Forexample, in the case of the 4:2:0 format, 67 intra prediction modes maybe used for the luma component to perform intra prediction and 35 intraprediction modes may be used for the chroma component. In the case ofthe 4:4:4 format, 67 intra prediction modes may be used for both theluma component and the chroma component to perform intra prediction.

Alternatively, depending on the size and/or shape of the block, adifferent number of intra prediction modes may be used to perform intraprediction. That is, depending on the size and/or shape of the PU or CU,35 intra prediction modes or 67 intra prediction modes may be used toperform intra prediction. For example, when the CU or PU has the sizeless than 64×64 or is asymmetrically partitioned, 35 intra predictionmodes may be used to perform intra prediction. When the size of the CUor PU is equal to or greater than 64×64, 67 intra prediction modes maybe used to perform intra prediction. 65 directional intra predictionmodes may be allowed for Intra_2N×2N, and only 35 directional intraprediction modes may be allowed for Intra_N×N.

A size of a block to which the extended intra prediction mode is appliedmay be set differently for each sequence, picture or slice. For example,it is set that the extended intra prediction mode is applied to a block(e.g., CU or PU) which has a size greater than 64×64 in the first slice.On the other hands, it is set that the extended intra prediction mode isapplied to a block which has a size greater than 32×32 in the secondslice. Information representing a size of a block to which the extendedintra prediction mode is applied may be signaled through in units of asequence, a picture, or a slice. For example, the information indicatingthe size of the block to which the extended intra prediction mode isapplied may be defined as ‘log 2_extended_intra_mode_size_minus4’obtained by taking a logarithm of the block size and then subtractingthe integer 4. For example, if a value of log2_extended_intra_mode_size_minus4 is 0, it may indicate that theextended intra prediction mode may be applied to a block having a sizeequal to or greater than 16×16. And if a value of log2_extended_intra_mode_size_minus4 is 1, it may indicate that theextended intra prediction mode may be applied to a block having a sizeequal to or greater than 32×32.

As described above, the number of intra prediction modes may bedetermined in consideration of at least one of a color component, achroma format, and a size or a shape of a block. In addition, the numberof intra prediction mode candidates (e.g., the number of MPMs) used fordetermining an intra prediction mode of a current block to beencoded/decoded may also be determined according to at least one of acolor component, a color format, and the size or a shape of a block. Amethod of determining an intra prediction mode of a current block to beencoded/decoded and a method of performing intra prediction using thedetermined intra prediction mode will be described with the drawings.

FIG. 10 is a flowchart briefly illustrating an intra prediction methodaccording to an embodiment of the present invention.

Referring to FIG. 10, an intra prediction mode of the current block maybe determined at step S1000.

Specifically, the intra prediction mode of the current block may bederived based on a candidate list and an index. Here, the candidate listcontains multiple candidates, and the multiple candidates may bedetermined based on an intra prediction mode of the neighboring blockadjacent to the current block. The neighboring block may include atleast one of blocks positioned at the top, the bottom, the left, theright, and the corner of the current block. The index may specify one ofthe multiple candidates of the candidate list. The candidate specifiedby the index may be set to the intra prediction mode of the currentblock.

An intra prediction mode used for intra prediction in the neighboringblock may be set as a candidate. Also, an intra prediction mode havingdirectionality similar to that of the intra prediction mode of theneighboring block may be set as a candidate. Here, the intra predictionmode having similar directionality may be determined by adding orsubtracting a predetermined constant value to or from the intraprediction mode of the neighboring block. The predetermined constantvalue may be an integer, such as one, two, or more.

The candidate list may further include a default mode. The default modemay include at least one of a planar mode, a DC mode, a vertical mode,and a horizontal mode. The default mode may be adaptively addedconsidering the maximum number of candidates that can be included in thecandidate list of the current block.

The maximum number of candidates that can be included in the candidatelist may be three, four, five, six, or more. The maximum number ofcandidates that can be included in the candidate list may be a fixedvalue preset in the device for encoding/decoding a video, or may bevariably determined based on a characteristic of the current block. Thecharacteristic may mean the location/size/shape of the block, thenumber/type of intra prediction modes that the block can use, a colortype, a color format, etc. Alternatively, information indicating themaximum number of candidates that can be included in the candidate listmay be signaled separately, and the maximum number of candidates thatcan be included in the candidate list may be variably determined usingthe information. The information indicating the maximum number ofcandidates may be signaled in at least one of a sequence level, apicture level, a slice level, and a block level.

When the extended intra prediction modes and the 35 pre-defined intraprediction modes are selectively used, the intra prediction modes of theneighboring blocks may be transformed into indexes corresponding to theextended intra prediction modes, or into indexes corresponding to the 35intra prediction modes, whereby candidates can be derived. For transformto an index, a pre-defined table may be used, or a scaling operationbased on a predetermined value may be used. Here, the pre-defined tablemay define a mapping relation between different intra prediction modegroups (e.g., extended intra prediction modes and 35 intra predictionmodes).

For example, when the left neighboring block uses the 35 intraprediction modes and the intra prediction mode of the left neighboringblock is 10 (a horizontal mode), it may be transformed into an index of16 corresponding to a horizontal mode in the extended intra predictionmodes.

Alternatively, when the top neighboring block uses the extended intraprediction modes and the intra prediction mode the top neighboring blockhas an index of 50 (a vertical mode), it may be transformed into anindex of 26 corresponding to a vertical mode in the 35 intra predictionmodes.

Based on the above-described method of determining the intra predictionmode, the intra prediction mode may be derived independently for each ofthe luma component and the chroma component, or the intra predictionmode of the chroma component may be derived depending on the intraprediction mode of the luma component.

Specifically, the intra prediction mode of the chroma component may bedetermined based on the intra prediction mode of the luma component asshown in the following Table 1.

TABLE 1 IntraPredModeY[xCb][yCb] Intra_chroma_pred_mode[xCb][yCb] 0 2610 1 X(0 <= X <= 34) 0 34 0 0 0 0 1 26 34 26 26 26 2 10 10 34 10 10 3 11 1 34 1 4 0 26 10 1 X

In Table 1, intra_chroma_pred_mode means information signaled to specifythe intra prediction mode of the chroma component, and IntraPredModeYindicates the intra prediction mode of the luma component.

Referring to FIG. 10, a reference sample for intra prediction of thecurrent block may be derived at step S1010.

Specifically, a reference sample for intra prediction may be derivedbased on a neighboring sample of the current block. The neighboringsample may be a reconstructed sample of the neighboring block, and thereconstructed sample may be a reconstructed sample before an in-loopfilter is applied or a reconstructed sample after the in-loop filter isapplied.

A neighboring sample reconstructed before the current block may be usedas the reference sample, and a neighboring sample filtered based on apredetermined intra filter may be used as the reference sample.Filtering of neighboring samples using an intra filter may also bereferred to as reference sample smoothing. The intra filter may includeat least one of the first intra filter applied to multiple neighboringsamples positioned on the same horizontal line and the second intrafilter applied to multiple neighboring samples positioned on the samevertical line. Depending on the positions of the neighboring samples,one of the first intra filter and the second intra filter may beselectively applied, or both intra filters may be applied. At this time,at least one filter coefficient of the first intra filter or the secondintra filter may be (1, 2, 1), but is not limited thereto.

Filtering may be adaptively performed based on at least one of the intraprediction mode of the current block and the size of the transform blockfor the current block. For example, when the intra prediction mode ofthe current block is the DC mode, the vertical mode, or the horizontalmode, filtering may not be performed. When the size of the transformblock is N×M, filtering may not be performed. Here, N and M may be thesame values or different values, or may be values of 4, 8, 16, or more.For example, if the size of the transform block is 4×4, filtering maynot be performed. Alternatively, filtering may be selectively performedbased on the result of a comparison of a pre-defined threshold and thedifference between the intra prediction mode of the current block andthe vertical mode (or the horizontal mode). For example, when thedifference between the intra prediction mode of the current block andthe vertical mode is greater than a threshold, filtering may beperformed. The threshold may be defined for each size of the transformblock as shown in Table 2.

TABLE 2 8 × 8 transform 16 × 16 transform 32 × 32 transform Threshold 71 0

The intra filter may be determined as one of multiple intra filtercandidates pre-defined in the device for encoding/decoding a video. Tothis end, an index specifying an intra filter of the current block amongthe multiple intra filter candidates may be signaled. Alternatively, theintra filter may be determined based on at least one of the size/shapeof the current block, the size/shape of the transform block, informationon the filter strength, and variations of the neighboring samples.

Referring to FIG. 10, intra prediction may be performed using the intraprediction mode of the current block and the reference sample at stepS1020.

That is, the prediction sample of the current block may be obtainedusing the intra prediction mode determined at step S500 and thereference sample derived at step S510. However, in the case of intraprediction, a boundary sample of the neighboring block may be used, andthus quality of the prediction picture may be decreased. Therefore, acorrection process may be performed on the prediction sample generatedthrough the above-described prediction process, and will be described indetail with reference to FIGS. 11 to 13. However, the correction processis not limited to being applied only to the intra prediction sample, andmay be applied to an inter prediction sample or the reconstructedsample.

FIG. 11 is a diagram illustrating a method of correcting a predictionsample of a current block based on differential information ofneighboring samples according to an embodiment of the present invention.

The prediction sample of the current block may be corrected based on thedifferential information of multiple neighboring samples for the currentblock. The correction may be performed on all prediction samples in thecurrent block, or may be performed on prediction samples inpredetermined partial regions. The partial regions may be one row/columnor multiple rows/columns, and these may be preset regions for correctionin the device for encoding/decoding a video. For example, correction maybe performed on a one row/column located at a boundary of the currentblock or may be performed on plurality of rows/columns from a boundaryof the current block. Alternatively, the partial regions may be variablydetermined based on at least one of the size/shape of the current blockand the intra prediction mode.

The neighboring samples may belong to the neighboring blocks positionedat the top, the left, and the top left corner of the current block. Thenumber of neighboring samples used for correction may be two, three,four, or more. The positions of the neighboring samples may be variablydetermined depending on the position of the prediction sample which isthe correction target in the current block. Alternatively, some of theneighboring samples may have fixed positions regardless of the positionof the prediction sample which is the correction target, and theremaining neighboring samples may have variable positions depending onthe position of the prediction sample which is the correction target.

The differential information of the neighboring samples may mean adifferential sample between the neighboring samples, or may mean a valueobtained by scaling the differential sample by a predetermined constantvalue (e.g., one, two, three, etc.). Here, the predetermined constantvalue may be determined considering the position of the predictionsample which is the correction target, the position of the column or rowincluding the prediction sample which is the correction target, theposition of the prediction sample within the column or row, etc.

For example, when the intra prediction mode of the current block is thevertical mode, differential samples between the top left neighboringsample p(−1, −1) and neighboring samples p (−1, y) adjacent to the leftboundary of the current block may be used to obtain the final predictionsample as shown in Equation 1.P′(0,y)=P(0,y)+((p(−1,y)−p(−1,−1))>>1 for y=0 . . . N−1  [Equation 1]

For example, when the intra prediction mode of the current block is thehorizontal mode, differential samples between the top left neighboringsample p(−1, −1) and neighboring samples p(x, −1) adjacent to the topboundary of the current block may be used to obtain the final predictionsample as shown in Equation 2.P′(x,0)=p(x,0)+((p(x,−1)−p(−1,−1))>>1 for x=0 . . . N−2  [Equation 2]

For example, when the intra prediction mode of the current block is thevertical mode, differential samples between the top left neighboringsample p(−1, −1) and neighboring samples p(−1, y) adjacent to the leftboundary of the current block may be used to obtain the final predictionsample. Here, the differential sample may be added to the predictionsample, or the differential sample may be scaled by a predeterminedconstant value, and then added to the prediction sample. Thepredetermined constant value used in scaling may be determineddifferently depending on the column and/or row. For example, theprediction sample may be corrected as shown in Equation 3 and Equation4.P′(0,y)=P(0,y)+((p(−1,y)−p(−1,−1))>>1 for y=0 . . . N−1  [Equation 3]P′(1,y)=P(1,y)+((p(−1,y)−p(−1,−1))>>2 for y=0 . . . N−1  [Equation 4]

For example, when the intra prediction mode of the current block is thehorizontal mode, differential samples between the top left neighboringsample p(−1, −1) and neighboring samples p(x, −1) adjacent to the topboundary of the current block may be used to obtain the final predictionsample, as described in the case of the vertical mode. For example, theprediction sample may be corrected as shown in Equation 5 and Equation6.P′(x0,)=p(x0,)+((p(x,−1)−p(−1,−1))>>1 for x=0 . . . N−1  [Equation 5]P′(x,1)=p(x,1)+((p(x,−1)−p(−1,−1))>>2 for x=0 . . . N−1  [Equation 6]

FIGS. 12 and 13 are diagrams illustrating a method of correcting aprediction sample based on a predetermined correction filter accordingto an embodiment of the present invention.

The prediction sample may be corrected based on the neighboring sampleof the prediction sample which is the correction target and apredetermined correction filter. Here, the neighboring sample may bespecified by an angular line of the directional prediction mode of thecurrent block, or may be at least one sample positioned on the sameangular line as the prediction sample which is the correction target.Also, the neighboring sample may be a prediction sample in the currentblock, or may be a reconstructed sample in a neighboring blockreconstructed before the current block.

At least one of the number of taps, strength, and a filter coefficientof the correction filter may be determined based on at least one of theposition of the prediction sample which is the correction target,whether or not the prediction sample which is the correction target ispositioned on the boundary of the current block, the intra predictionmode of the current block, angle of the directional prediction mode, theprediction mode (inter or intra mode) of the neighboring block, and thesize/shape of the current block.

Referring to FIG. 12, when the directional prediction mode has an indexof 2 or 34, at least one prediction/reconstructed sample positioned atthe bottom left of the prediction sample which is the correction targetand the predetermined correction filter may be used to obtain the finalprediction sample. Here, the prediction/reconstructed sample at thebottom left may belong to a previous line of a line including theprediction sample which is the correction target. Theprediction/reconstructed sample at the bottom left may belong to thesame block as the current sample, or to neighboring block adjacent tothe current block.

Filtering for the prediction sample may be performed only on the linepositioned at the block boundary, or may be performed on multiple lines.The correction filter where at least one of the number of filter tapsand a filter coefficient is different for each of lines may be used. Forexample, a (½, ½) filter may be used for the left first line closest tothe block boundary, a ( 12/16, 4/16) filter may be used for the secondline, a ( 14/16, 2/16) filter may be used for the third line, and a (15/16, 1/16) filter may be used for the fourth line.

Alternatively, when the directional prediction mode has an index of 3 to6 or 30 to 33, filtering may be performed on the block boundary as shownin FIG. 13, and a 3-tap correction filter may be used to correct theprediction sample. Filtering may be performed using the bottom leftsample of the prediction sample which is the correction target, thebottom sample of the bottom left sample, and a 3-tap correction filterthat takes as input the prediction sample which is the correctiontarget. The position of neighboring sample used by the correction filtermay be determined differently based on the directional prediction mode.The filter coefficient of the correction filter may be determineddifferently depending on the directional prediction mode.

Different correction filters may be applied depending on whether theneighboring block is encoded in the inter mode or the intra mode. Whenthe neighboring block is encoded in the intra mode, a filtering methodwhere more weight is given to the prediction sample may be used,compared to when the neighboring block is encoded in the inter mode. Forexample, in the case of that the intra prediction mode is 34, when theneighboring block is encoded in the inter mode, a (½, ½) filter may beused, and when the neighboring block is encoded in the intra mode, a (4/16, 12/16) filter may be used.

The number of lines to be filtered in the current block may varydepending on the size/shape of the current block (e.g., the coding blockor the prediction block). For example, when the size of the currentblock is equal to or less than 32×32, filtering may be performed on onlyone line at the block boundary; otherwise, filtering may be performed onmultiple lines including the one line at the block boundary.

FIGS. 12 and 13 are based on the case where the 35 intra predictionmodes in FIG. 7 are used, but may be equally/similarly applied to thecase where the extended intra prediction modes are used.

FIG. 14 shows a range of reference samples for intra predictionaccording to an embodiment to which the present invention is applied.

Referring to FIG. 14, intra prediction may performed by using referencesamples P (−1, −1), P (−1, y) (0<=y<=2N−1) and P (x, −1) (0<=x<=2N−1)located at a boundary of a current block. At this time, filtering onreference samples is selectively performed based on at least one of anintra prediction mode (e.g., index, directionality, angle, etc. of theintra prediction mode) of the current block or a size of a transformblock related to the current block.

Filtering on reference samples may be performed using an intra filterpre-defined in an encoder and a decoder. For example, an intra filterwith a filter coefficient of (1,2,1) or an intra filter with a filtercoefficient of (2,3,6,3,2) may be used to derive final reference samplesfor use in intra prediction.

Alternatively, at least one of a plurality of intra filter candidatesmay be selected to perform filtering on reference samples. Here, theplurality of intra filter candidates may differ from each other in atleast one of a filter strength, a filter coefficient or a tap number(e.g., a number of filter coefficients, a filter length). A plurality ofintra filter candidates may be defined in at least one of a sequence, apicture, a slice, or a block level. That is, a sequence, a picture, aslice, or a block in which the current block is included may use thesame plurality of intra filter candidates.

Hereinafter, for convenience of explanation, it is assumed that aplurality of intra filter candidates includes a first intra filter and asecond intra filter. It is also assumed that the first intra filter is a(1,2,1) 3-tap filter and the second intra filter is a (2,3,6,3,2) 5-tapfilter.

When reference samples are filtered by applying a first intra filter,the filtered reference samples may be derived as shown in Equation 7.P(−1,−1)=(P(−1,0)+2P(−1,−1)+P(0,−1)+2)>>2P(−1,y)=(P(−1,y+1)+2P)(−1,y)+P(−1,y−1)+2)>>2P(x,−1)=(P(x+1,−1)+2P(x,−1)+P(x−1,−1)+2)>>2  [Equation 7]

When reference samples are filtered by applying the second intra filter,the filtered reference samples may be derived as shown in the followingequation 8.P(−1,−1)=(2P(−2,0)+3P(−1,0)+6P(−1,−1)+3P(0,−1)+2P(0,−2)+8)>>4P(−1,y)=(2P(−1,y+2)+3P)(−1,y+1)+6P(−1,y)+3P(−1,y−1)+2P(−1,y−2)+8)>>4P(x,−1)=(2P(x+2,−1)+3P(x+1,−1)+6P(x,−1)+3P(x−1,−1)+2P(x−2,−1)+8)>>4  [Equation8]

In the above Equations 7 and 8, x may be an integer between 0 and 2N−2,and y may be an integer between 0 and 2N−2.

Alternatively, based on a position of a reference sample, one of aplurality of intra filter candidates may be determined, and filtering onthe reference sample may be performed by using the determined one. Forexample, a first intra filter may be applied to reference samplesincluded in a first range, and a second intra filter may be applied toreference samples included in a second range. Here, the first range andthe second range may be distinguished based on whether they are adjacentto a boundary of a current block, whether they are located at a top sideor a left side of a current block, or whether they are adjacent to acorner of a current block. For example, as shown in FIG. 15, filteringon reference samples (P (−1, −1), P (−1,0), P (−1,1), . . . , P (−1,N−1) and P (0, −1), P (1, −1), . . . ) which are adjacent to a boundaryof the current block is performed by applying a first intra filter asshown in Equation 7, and filtering on the other reference samples whichare not adjacent to a boundary of the current block is performed byapplying a second reference filter as shown in Equation 8. It ispossible to select one of a plurality of intra filter candidates basedon a transform type used for a current block, and perform filtering onreference samples using the selected one. Here, the transform type maymean (1) a transform scheme such as DCT, DST or KLT, (2) a transformmode indicator such as a 2D transform, 1D transform or non-transform or(3) the number of transforms such as a first transform and a secondtransform. Hereinafter, for convenience of description, it is assumedthat the transform type means the transform scheme such as DCT, DST andKLT.

For example, if a current block is encoded using a DCT, filtering may beperformed using a first intra filter, and if a current block is encodedusing a DST, filtering may be performed using a second intra filter. Or,if a current block is encoded using DCT or DST, filtering may beperformed using a first intra filter, and if the current block isencoded using a KLT, filtering may be performed using a second intrafilter.

Filtering may be performed using a filter selected based on a transformtype of a current block and a position of a reference sample. Forexample, if a current block is encoded using the a DCT, filtering onreference samples P (−1, −1), P (−1,0), P (−1,1), . . . , P (−1, N−1)and P (0, −1), P (1, −1), . . . , P (N−1, −1) may be performed by usinga first intra filter, and filtering on other reference samples may beperformed by using a second intra filter. If a current block is encodedusing a DST, filtering on reference samples P (−1, −1), P (−1,0), P(−1,1), . . . , P (−1, N−1) and P (0, −1), P (1, −1), . . . , P (N−1,−1) may be performed by using a second intra filter, and filtering onother reference samples may be performed by using a first intra filter.

One of a plurality of intra filter candidates may be selected based onwhether a transform type of a neighboring block including a referencesample is the same as a transform type of a current block, and thefiltering may be performed using the selected intra filter candidate.For example, when a current block and a neighboring block use the sametransform type, filtering is performed using a first intra filter, andwhen transform types of a current block and of a neighboring block aredifferent from each other, the second intra filter may be used toperform filtering.

It is possible to select any one of a plurality of intra filtercandidates based on a transform type of a neighboring block and performfiltering on a reference sample using the selected one. That is, aspecific filter may be selected in consideration of a transform type ofa block in which a reference sample is included. For example, as shownin FIG. 16, if a block adjacent to left/lower left of a current block isa block encoded using a DCT, and a block adjacent to top/top right of acurrent block is a block encoded using a DST, filtering on referencesamples adjacent to left/lower left of a current block is performed byapplying a first intra filter and filtering on reference samplesadjacent to top/top right of a current block is performed by applying asecond intra filter.

In units of a predetermined region, a filter usable in the correspondingregion may be defined. Herein, the unit of the predetermined region maybe any one of a sequence, a picture, a slice, a block group (e.g., a rowof coding tree units) or a block (e.g., a coding tree unit) Or, anotherregion may be defined that shares one or more filters. A referencesample may be filtered by using a filter mapped to a region in which acurrent block is included.

For example, as shown in FIG. 17, it is possible to perform filtering onreference samples using different filters in CTU units. In this case,information indicating whether the same filter is used in a sequence ora picture, a type of filter used for each CTU, an index specifying afilter used in the corresponding CTU among an available intra filtercandidates may be signaled via a sequence parameter set (SPS) or apicture parameter set (PPS).

The above-described intra filter may be applied in units of a codingunit. For example, filtering may be performed by applying a first intrafilter or a second intra filter to reference samples around a codingunit.

When an intra-prediction mode of a current block is determined,intra-prediction may be performed using a reference sample adjacent tothe current block. For example, prediction samples of a current blockmay be generated by averaging reference samples, or may be generated byduplicating reference samples in a specific direction considering adirectionality of an intra-prediction mode. As described above in anexample referring to FIG. 14, P(−1,−1), P(−1, y) (0<=y<=2N−1), P(x, −1)(0<=x<=2N−1) which are located at a boundary of a current block may beused as reference samples.

When it is determined that a sample included in a neighboring blockadjacent to a current block is not available as a reference sample, thesample that is not available may be replaced with a reference samplethat is available. For example, a neighboring sample may be determinedas unavailable in case where a position of a sample included in aneighboring block is outside a picture, a sample included in aneighboring block is present in a slice different from a current block,or a sample included in a neighboring block is included in a blockencoded by an inter-prediction. Here, whether or not a sample includedin a block encoded by an inter-prediction is unavailable may bedetermined based on information indicating whether to use a sampleincluded in a block encoded by an inter-prediction as a reference samplewhen performing intra-prediction of a current block. Here, theinformation may be a 1-bit flag (e.g.,‘constrained_intra_prediction_flag’), but is not limited thereto. Forexample, when a value of ‘constrained_intra_prediction_flag’ is 1, asample included in a block encoded by an inter-prediction may bedetermined to be unavailable as a reference sample. Hereinafter, asample that cannot be used as a reference sample will be referred to asa unavailable reference sample.

In the example shown in FIG. 14, when it is determined that a samplelocated at left lowermost (e.g., P(−1, 1, 2N−1)) is not available, thesample located at left lowermost may be replaced with a first availablereference sample which is firstly searched by scanning available samplesin a predetermined order. Here, the scanning order may be sequentiallyperformed from a sample adjacent to the left lowermost sample. Forexample, in the example shown in FIG. 14, when a sample P(−1, 2N−1) isnot available, scanning may be performed in an order of P(−1, −2N−2) toP(−1, −1), P(−1) to P(2N−1, −1). P(−1, 2N−1) may be replaced with afirst available reference sample that is found as a result of the scan.

When a left reference sample except for a reference sample located atleft lowermost is unavailable, the left reference sample may be replacedwith a reference sample adjacent to a bottom of the left referencesample. For example, an unavailable reference sample P(−1, y) betweenP(−1, 2N−1) and P(−1, −1) may be replaced with a reference sample P(−1,y+1).

When a top reference sample is unavailable, the top reference sample maybe replaced with a reference sample adjacent to a left of the topreference sample. For example, an unavailable reference sample P(x, −1)between P(0, −1) and P(2N−1, −1) may be replaced with a reference sampleP(x−1, −1).

A reference sample set for a current block may be referred to as a‘reference line’ (or ‘intra-reference line’ or ‘reference sample line’).Here, the reference line may include a set of reference samples composedof one row and one column. For example, in the example shown in FIG. 14,a ‘reference line’ a reference sample set including P(−1, 2N−1) to P(−1,1), P(0, −1) to P(2N−2, −1). An intra-prediction of a current block maybe performed based on reference samples included in a reference line. Anintra-prediction of a current block may be performed, using referencesamples included in a reference line, based on an intra-prediction modeof a current block, For example, when an intra-prediction mode of acurrent block is a DC mode, a prediction signal may be generated usingan average and weighted prediction of reference samples included in thereference line. For example, when an intra-prediction mode of a currentblock is a DC mode, prediction samples of the current block may beobtained according to Equation 9.P(0,0)=(P(−1,0)+P(0,−1)+2*dcVal)>>2P(x,0)=(P(x,−1)+3*dcVal)>>2P(0,y)=(P(−1y)+3*dcVal)>>2  [Equation 9]

In Equation 9, dcVal may be generated based on an average value ofsamples except for P(−1, −1) among reference samples included in areference line.

A planar mode provides effective prediction efficiency in a smooth areahaving no strong edges, and is effective in improving discontinuity ofblock boundary or image quality deterioration of a block boundary. Whenan intra-prediction mode of a current block is a planar mode, ahorizontal direction provisional prediction sample of the current blockmay be obtained using a reference sample adjacent to a top right cornerof the current block and a reference sample having y coordinateidentical to the horizontal direction provisional prediction sample, anda vertical direction provisional prediction sample of the current blockmay be obtained using a reference sample adjacent to a bottom leftcorner of the current block and a reference sample having x coordinateidentical to the vertical direction provisional prediction sample. Forexample, a horizontal direction provisional prediction sample and avertical direction provisional prediction sample of a current block maybe obtained by according to Equation 10.P _(h)(x,y)=(N−1−x)*P(−1,y)+(x+1)*P(N,−1)P _(v)(x,y)=(N−1−y)*P(x,−1)+(y+1)*P(−1,N)  Equation[10]

A prediction sample of a current block may be generated by summing ahorizontal direction provisional prediction sample and a verticaldirection provisional prediction sample, and then shifting the summationresult by a value determined according to a size of a current block. Forexample, a prediction sample of a current block may be obtainedaccording to Equation 11.P(x,y)=(P _(h)(x,y)+Pv(x,y)+N)>>(log 2(N)+1)  [Equation 11]

An intra-prediction of a current block may be performed using aplurality of reference lines. Lengths of the plurality of referencelines may be all or part of the same, or may be set different from eachother.

For example, assuming that a current block has a W×H size, k-threference line may include p(−k, −k), reference samples located in a rowidentical to p(−k, −k) (e.g., reference samples from p(k+1, −k) top(W+H+2(k−1), −k) or reference samples from p(−k+1, −k) to p(2W+2(k−1),−k)) and reference samples located in a column identical to p(−k, −k)(e.g., reference samples from p(−k, −k+1) to p(−k, W+H+2(k−1)) orreference samples from p(−k, −k+1) to p(−k, 2H+2(k−1))).

FIG. 18 exemplifies a plurality of reference sample lines. As in theexample shown in FIG. 18, when a first reference line adjacent to aboundary of a current block is referred to as a ‘reference line 0’, k-threference line may be set adjacent to (k−1)-th reference line.

Alternatively, unlike the example shown in FIG. 18, it is also possibleto configure all the reference lines to have the same number ofreference samples.

An intra-prediction of a current block may be performed by at least oneof a plurality of reference lines. A method of performingintra-prediction using a plurality of reference lines as described abovemay be referred to as an ‘intra-prediction method using an extendedreference sample’ or an ‘extended intra-prediction method.’ In addition,a plurality of reference lines can be referred to as an ‘extendedreference line’.

Whether or not performing intra-prediction using an extended referenceline may be determined based on information signaled through abitstream. Here, the information may be a 1-bit flag, but is not limitedthereto. Information on whether performing intra-prediction using anextended reference line may be signaled in units of a coding tree unit,an encoding unit or a prediction unit, or may be signaled in units of asequence, a picture or a slice. That is, whether to perform intraprediction using the extended reference line may be determined in unitsof a sequence, a picture, a slice, a CTU, a CU, or a PU.

Alternatively, whether or not performing intra-prediction using anextended reference line may be determined based on at least one of asize, shape, depth or intra-prediction mode of a current block.

For example, it may be determined whether to perform intra predictionusing an extended reference line, depending on whether a current blockhas a square shape or a non-square shape. For example, if the currentblock has the square shape, intra prediction of the current block may beperformed using the extended reference line, as in the example shown inFIG. 19A. That is, when the current block has the square shape, intraprediction may be performed using at least one of a plurality ofreference lines around the current block. On the other hand, when thecurrent block has the non-square shape, intra prediction of the currentblock may be performed without using the extended reference line, as inthe example shown in FIG. 19B. That is, when the current block has thenon-square shape, intra prediction may be performed using one referenceline adjacent to the current block.

In contrast to the example shown in FIGS. 19A and 19B, when the currentblock has the non-square shape, intra prediction may be performed usingan extended reference line, and when the current block has the squareshape, intra prediction may be performed without using the extendedreference line.

When it is determined to perform intra-prediction using an extendedreference line, a number of reference lines may be determined. Here, anumber of reference lines may have a fixed value, and may be adaptivelydetermined according to a size, shape or intra-prediction mode of acurrent block. For example, when an intra-prediction mode of a currentblock is a non-directional mode, intra-prediction of the current blockis performed using one reference line. When an intra-prediction mode ofa current block is a directional mode, intra-prediction of the currentblock may be performed using a plurality of reference lines.

For an additional example, a number of reference lines may be determinedby information that is signaled in units of a sequence, a picture, aslice or a unit to be decoded. Here, the unit to be decoded mayrepresents a coding tree unit, a coding unit, a transform unit, aprediction unit, or the like. For example, a syntax element‘max_intra_line_idx_minus2’ indicating a number of available referencelines available in a sequence or a slice may be signaled through asequence header or a slice header. In this case, the number of availablereference lines may be set to max_intra_line_idx_minus2+2.

Hereinafter, a method of performing intra-prediction using an extendedreference line will be described in detail.

FIG. 20 is a flowchart illustrating a method of performingintra-prediction using an extended reference line according to thepresent invention.

First, a decoder may generate a plurality of reference lines (S2010).Reference samples included in each reference line may be generated basedon reconstructed samples included in blocks decoded earlier than acurrent block.

When an intra-prediction mode of a current block is a directional mode,a decoder may generate a reference line considering a directionality ofthe intra-prediction mode. Considering a directionality of anintra-prediction mode, a larger number of reference samples may beincluded in k-th reference line than in (k−1)-th reference line. Thatis, a reference line away from a current block may include a largernumber of reference samples than a reference line near the currentblock.

Here, a number of reference samples further included in k-th referenceline than in (k−1)-th reference line may be variably determinedaccording to a size, a shape or an intra prediction mode of a currentblock.

For example, when a current block has a 4×4 size, k-th reference linemay further include four (specifically, 2 in horizontal direction and 2in vertical direction) reference samples than (k−1)-th reference line.In addition, when a current block has a size of 8×8, k-th reference linemay further include eight (specifically, 4 in horizontal direction and 4in vertical direction) reference samples than (k−1)-th reference line.

Referring to FIG. 18, as a size of a current block size is 4×4, it isexemplified that a first reference sample includes a total of 9reference samples and a second reference sample includes a total of 13(=9+2×2) reference samples.

When a current block is non-square, a number of reference samplesincluded in a reference line may be determined according to a horizontaland vertical lengths of a current block.

For an example, FIG. 21 is a diagram exemplifying a plurality ofreference lines for a non-square block. Describing with comparing FIGS.18 and 21, as a width of a current block decreases to ½, a number of topreference samples except for a top left reference sample included in areference line 0 is reduced from 8 to 4.

That is, according to FIGS. 15 and 17, when assuming that a currentblock has a W×H size, k-th reference line may include a total of2{(W+H)+2(k−1)}+1 reference samples including W+H+2(k−1) top referencesamples (or 2W+2(k−1) top reference samples) (i.e., horizontal directionreference samples), W+H+2(k−1) left reference samples (or 2H+2(k−1) leftreference samples) (i.e., vertical direction reference samples) and topleft reference sample.

If a reference sample that is not available is included in a referenceline, the unavailable reference sample may be replaced with aneighboring available reference sample. At this time, the neighboringsample replacing the unavailable reference sample may included in a samereference line as the unavailable reference sample, or may be includedin the reference line different from the unavailable reference sample.

For example, if a position of a reference sample is outside a picture orin a slice different from a current block when intra prediction isperformed using an extended reference line or if a reference sample isincluded in a block encoded by inter prediction when intra prediction isperformed using an extended reference line, the reference sample may bedetermined unavailable. The reference sample included in a block encodedby inter prediction may be determined unavailable when it is set that areference sample included in a block encoded by inter prediction is notused (e.g., only when a value of constrained_intra_prediction_flag is0). Or, if it is set that a block encoded by intra-prediction should bedecoded earlier than a block encoded by inter-prediction, the blockencoded by inter-prediction may not yet be reconstructed yet when theblock encoded by intra-prediction is decoded. Accordingly, a referencesample included in the block encoded by inter prediction may bedetermined unavailable.

A reference sample used for replacing an unavailable reference samplemay be determined in consideration of a position of the unavailablereference sample, a distance between the unavailable reference sampleand the available reference sample, or the like. For example, anunavailable sample may be replaced with an available sample which has ashortest distance from the unavailable reference sample. That is, anavailable reference sample which has the shortest distance and isselected by comparing a distance (first offset) between an availablereference sample included in the same reference line with theunavailable reference sample and the unavailable sample and a distance(second offset) between an available reference sample included in areference line different from the unavailable reference sample and theunavailable sample may be substituted for the unavailable referencesample. Alternatively, an available reference sample located in apredetermined direction from the unavailable reference sample mayreplace the unavailable reference sample. Here, the predetermineddirection may be a predefined direction (e.g., left, right, up or down)in the encoder/decoder. The predefined direction may be set differentlydepending on a position of the unavailable reference sample.

In the example shown in FIG. 22, it is depicted that a distance betweenthe unavailable reference sample included in the reference line 0 andthe available reference sample included in the reference line 0 is 4,and a distance between the unavailable reference sample included in thereference line 0 and the available reference sample included in thereference line 2 is 2. Accordingly, the unavailable sample included inthe reference line 0 may be substituted by using the available referencesample included in the reference line 2.

If a first offset and a second offset are the same, an unavailablereference sample may be replaced using an available reference sampleincluded in the same reference line as the unavailable reference sample.

An unavailable reference sample may be replaced using an availablereference sample included in a reference line different from theunavailable reference sample only when a distance (i.e., a first offset)between an available sample included in the same reference line as theunavailable reference sample and the unavailable reference sample isequal to or greater than N. Alternatively, even when the first offset isequal to or greater than N, an available reference sample included in areference line different from an unavailable reference sample may beused to replace the unavailable reference sample only when a secondoffset is smaller than the first offset. Here, N may represent aninteger of 1 or more.

If a first offset is not equal to or greater than N, an unavailablereference sample may be replaced using an available reference sampleincluded in the same reference line as the unavailable reference sample.

FIGS. 23 and 24 show an example in which an unavailable reference sampleis replaced with an available reference sample when N is 2. If adistance between an unavailable reference sample included in referenceline 0 and an available reference sample included in the reference line0 is 2, as in the example shown in FIG. 23, the unavailable referencesample included in the reference line 0 may be replaced using 1.

On the other hand, if a distance between an unavailable reference sampleincluded in reference line 0 and an available reference sample includedin the reference line 0 is 1, as in the example shown in FIG. 24, theunavailable reference sample included in the reference line 0 may bereplaced using the available reference sample included in the referenceline 0.

An unavailable reference sample may be replaced using an availablereference sample included in the same reference line as the unavailablereference sample or an available reference sample included in areference line adjacent to a reference line in which the unavailablereference sample is included. Here, a reference line adjacent to areference line in which the unavailable reference sample is included mayrefer to a reference line having an index difference of 1 from thereference line including the unavailable reference sample.Alternatively, the unavailable reference sample may be replaced with anavailable reference sample included in a reference line having an indexdifference of two or more from the reference line including theunavailable reference sample.

Alternatively, an unavailable reference sample may be replaced using anavailable reference sample included in a reference line having a largerindex value or having a smaller index value than a reference lineincluding the unavailable reference sample. For example, if a referenceline having a larger index value than a reference line including theunavailable reference sample is used, a reference sample located at aleft or a top of the unavailable reference sample may be used to replacethe unavailable reference sample.

A search for an available reference sample to replace an unavailablereference sample may be performed in a predefined direction. Forexample, only a reference sample located in a direction of either a top,a bottom, a left, or a right of the unavailable sample among referencesamples included in the same reference line as the unavailable referencesample may be used to replace the unavailable reference sample.Alternatively, only a reference sample located in a direction of eithera top, a bottom, a left, or a right of the unavailable sample amongreference samples included in a reference line different from theunavailable reference sample may be used to replace the unavailablesample.

A decoder may decode, based on a bitstream, index information specifyingat least one of a plurality of reference lines (S2020). For example,when 4 reference lines are available as in the example shown in FIG. 18,index information may specify at least one of the 4 reference lines.

A reference line for performing intra-prediction for a current block maybe adaptively determined based on a size of a current block, a type of acurrent block, an intra-prediction mode of a current block, indexinformation in a neighboring block or a difference between anintra-prediction mode of a current block and a predeterminedintra-prediction mode, and the like.

In addition, the number of reference lines used for intra prediction ofa current block may have a fixed value or may be adaptively determinedaccording to a size, a shape, or an intra prediction mode of the currentblock.

When at least one of a plurality of reference lines is determined, adecoder may perform intra-prediction for a current block using thedetermined reference line (S2030). In this case, a position of areference sample used in the intra prediction of the current block inthe selected reference line may be derived according to at least one ofa type of the intra prediction mode or a directionality of the intraprediction mode of the current block.

For example, when an intra prediction mode of a current block is DCmode, a prediction sample of the current block may be generated based onan average value (dcVal) of all or a part of reference samples includedin the determined reference line. Referring to FIGS. 25 and 26, acalculation of the average value of the reference samples included inthe reference line will be described in detail.

Alternatively, when an intra-prediction mode of a current block is adirectional mode, a prediction sample of the current block may begenerated based on a reference sample specified by the directional modeamong reference samples included in the determined reference line. Atthis time, if a line segment extending from a prediction sample towardthe direction indicated by the directional mode points between referencesamples, the prediction sample of the current block may be generatedbased on a weighted sum (weighted prediction) of a first referencesample and a second reference sample which are located at both sides ofthe point indicated by the line segment extending toward the directionindicated by the directional mode.

When an intra prediction mode of a current block is DC mode, there isneed to calculate an average value (dcVal) of reference samples includedin a reference line in order to perform prediction on the current block.At this time, the average value for reference samples in k-th referenceline may be calculated using only a part of reference samples includedin the k-th reference line. At this time, the number of referencesamples used to derive the average value may be the same for eachreference line, or may be different for each reference line.

Alternatively, an average value for reference samples in k-th referenceline may be derived using all of reference samples included in the k-threference line. Alternatively, it may be determined based on a size of acurrent block, a shape of a current block, or a position of a referenceline whether to derive an average value using a part of referencesamples in k-th reference line or to derive an average value using allof reference samples in k-th reference line.

FIG. 25 is a diagram illustrating reference samples used to derive anaverage value of a reference line.

FIG. 25 shows an example of deriving a reference sample average value ofk-th reference line using a part of reference samples included in areference line. For example, an example illustrated in FIG. 25, areference sample average value of a first reference line adjacent to acurrent block (i.e., reference line 0 shown in FIG. 25) may becalculated using top reference samples and left reference samplesexcluding a reference sample adjacent to a left-top corner of thecurrent block. That is, when a size of the current block is N×N, a totalof 4N reference samples such as 2N top reference samples and 2N leftreference samples may be used for calculation of the average value ofthe first reference line.

The number of reference samples used to calculate a reference sampleaverage value of k-th reference line may be same as the number ofreference samples used to calculate a reference sample average value ofthe first reference line. At this time, a position of a reference sampleused to calculate the average value of the k-th reference line maycorrespond to a position of a reference sample used to calculate thereference sample average value of the first reference line.

A reference sample in k-th reference line corresponding to a referencesample of a first reference line may have the same x-coordinate or thesame y-coordinate as the reference sample of the first reference line.For example, a coordinate of a top reference sample included in k-threference line corresponding to a top reference sample P (i, j) includedin a first reference line may be P(i,j−k+1) which has the same xcoordinate as P(i,j). For example, a coordinate of a left referencesample in k-th reference line corresponding to a left reference sample P(i, j) included in a first reference line may be P(i−k+1,j) which hasthe same y coordinate as P(i,j).

In FIG. 25, reference samples of second to fourth reference linescorresponding to upper reference sample and left reference sample in afirst reference line are shown. A reference sample average value of eachreference line may be calculated using the reference samples shown inFIG. 25.

In FIG. 25, it is assumed that a current block has a square shape, buteven if the current block has a non-square shape, the above embodimentcan be applied as it is. For example, when the current is a non-squareblock having W×H size, a reference sample average value of eachreference line may be calculated using a total of 2(W+H) referencesamples, such as 2W top reference samples and 2H left reference samples.Accordingly, as in the example shown in FIG. 26, the number of referencesamples used for calculating an average value of the k-th reference linemay have the same value as the number of reference samples used forcalculating an average value of the first reference line. Also, thelocation of the reference sample used to calculate the average value ofthe k-th reference line may correspond to the location of the referencesample used to calculate the reference sample average value of the firstreference line.

In FIGS. 25 and 26, top reference samples as much as twice a width of acurrent block and left reference samples as much as twice a height ofthe current block are used to calculate a reference sample average valueof a reference line. A reference sample average value of a referenceline may be calculated using fewer or larger number of reference samplesthan those shown in FIGS. 25 and 26. Fig. For example, the referencesample average value of the reference line may be calculated using thesame number of top reference samples as the width of the current blockand the same number of left reference samples as the height of thecurrent block.

A reference sample average value of a reference line may be calculatedby assigning different weights to reference samples, depending on ashape of a current block and a position of a reference sample. Forexample, if the current block has a square shape, the reference sampleaverage value may be calculated by assigning the same weight to topreference samples and left reference samples. On the other hand, whenthe current block has a non-square shape, the reference sample averagevalue may be calculated by assigning a larger weight to one of topreference samples and left reference samples. For example, if a heightof the current block is larger than a width, the average value may becalculated by assigning a larger weight to the top reference samplesthan the left reference samples. On the other hand, when the width ofthe current block is larger than the height, the average value may becalculated by assigning a larger weight to the left reference samplesthan the top reference samples.

For example, when the size of the current block is N/2×N, the averagevalue of the k-th reference line dcVal may be calculated by thefollowing Equation 12.

$\begin{matrix}{{dcVal} = {\left( {\sum\limits_{l = 0}^{{2N} - 1}{P\left( {{- k},l} \right)}} \right) \gg {{2N} + \left( {\sum\limits_{l = 0}^{{2N} - 1}{2 \times {P\left( {l,{- k}} \right)}}} \right)} \gg {2N}}} & \left\lbrack {{Equation}12} \right\rbrack\end{matrix}$

For example, when the size of the current block is N×N/2, the averagevalue of the k-th reference line dcVal may be calculated by thefollowing Equation 13.

$\begin{matrix}{{dcVal} = {\left( {\sum\limits_{l = 0}^{{2N} - 1}{2 \times {P\left( {{- k},l} \right)}}} \right) \gg {{2N} + \left( {\sum\limits_{l = 0}^{{2N} - 1}{P\left( {l,{- k}} \right)}} \right)} \gg {2N}}} & \left\lbrack {{Equation}13} \right\rbrack\end{matrix}$

In Equations 12 and 13, k may be set to a value between 1 andmax_intra_line_idx_minus2+2.

As in the above example, the average value may be calculated usingreference samples corresponding to reference samples included in thefirst intra reference line among reference samples of the k-th intrareference line. At this time, the average value may be calculated basedon a predetermined weight. Here, the weight may be derived based on adistance of the first and k-th intra reference lines from the currentblock.

In the example described through FIG. 20, it is exemplified that indexinformation specifying one of the plurality of reference lines isdecoded after generating a plurality of reference lines. It is alsopossible to obtain only a reference line specified by index informationamong a plurality of reference lines after decoding the indexinformation specifying one of the plurality of reference lines.

In the embodiment described through FIG. 20, it is described thatintra-prediction for a current block is performed using at least onereference line specified by index information among a plurality ofreference lines. That is, intra-prediction for a current block may beperformed using one reference line or two or more reference lines.Whether or not to use two or more reference lines in performingintra-prediction for a current block may be determined based oninformation signaled from a bitstream, a size of a current block, a typeof a current block, an intra-prediction mode of a current block, whetheran intra-prediction mode of a current block is a non-directional or adifference between an intra-prediction mode of a current block and apredetermined intra-prediction mode, and the like.

The two or more reference lines may be specified by a plurality of indexinformation signaled from a bitstream. For example, when two referencelines are set to be used, any one of the two reference lines may bespecified by first index information, and the other may be specified bysecond index information.

Alternatively, two or more reference lines may be spatially contiguous.In this case, index information for specifying any one of the two ormore reference lines may be signaled through a bitstream. If any one ofthe two or more reference lines is selected by the index information,the remaining reference line may be automatically selected based on thespatial adjacency with the selected reference line. For example, when itis set to use two reference lines, and index information indicates‘reference line 0,’ then intra-prediction of a current block may beperformed based on reference line 0 and reference line 1 neighboring thereference line 0.

When it is set to use a plurality of reference lines, intra-predictionof a current block may be performed based on an average value, a maximumvalue, a minimum value or a weighted sum of reference samples includedin the plurality of reference lines.

For example, assuming that an intra-prediction mode of a current blockis a directional mode (i.e., an Angular mode), a predicted sample of thecurrent block may be generated based on a first reference sample and asecond reference sample, each of which is included in a differencereference line. Here, a first reference line including the firstreference sample and a second reference line including the secondreference sample may be positioned neighboring each other, but it is notlimited thereto. In addition, the first reference sample and the secondreference sample may be determined by an intra prediction mode of thecurrent block. The first reference sample and the second referencesample may be positioned neighboring each other, but it is not limitedthereto. A prediction sample of a current block may be generated inconsideration of a weighted sum of the first reference sample and thesecond reference sample, or may be generated based on an average value,a minimum value or a maximum value of the first reference sample and thesecond reference sample.

Intra-prediction of a current block may be performed by performing afirst intra-prediction based on a part of a plurality of reference linesand performing a second intra-prediction based on the remainingreference lines. Here, an intra-prediction mode used in a firstintra-prediction and an intra-prediction mode used in a secondintra-prediction may be the same or different. A prediction sample of acurrent block may be generated based on a first prediction samplegenerated by performing a first intra-prediction and a second predictionsample generated by performing a second intra-prediction.

Reference sample smoothing may be performed on reference samplesincluded in a reference line. That is, at least one of a plurality offilters may be used to filter the reference samples included in thereference line. Intra prediction of the current block may be performedbased on unfiltered reference sample or filtered reference sample.

Reference sample smoothing may be performed on all of a plurality ofreference lines or on a part of a plurality of reference lines. Forexample, the reference sample smoothing may be performed on a referenceline that is used for intra prediction of the current block among aplurality of reference lines, for example, a reference line specified byindex information.

It may be adaptively determined based on at least one of a size of acurrent block, a type of an intra prediction mode, a directionality ofan intra prediction mode, a position of a reference line, a number ofreference lines or a variation between reference lines whether toperform reference sample smoothing. The variation between referencelines may represent a difference value between reference samplesincluded in different reference lines. For example, if the size of theprediction block for the current block is 4×4, or if the intraprediction mode of the current block is DC mode, reference samplesmoothing may be omitted.

In addition, the number of times in which a filter is applied or a typeof a filter may be adaptively determined depending on at least one of asize of a current block, a type of an intra prediction mode, adirectionality of an intra prediction mode, a position of a referenceline, a number of reference lines, or a variation between referencelines. For example, when the size of a prediction block related to acurrent block is equal to or less than 32×32 and the intra predictionmode of the current block does not have a horizontal direction, similarto the horizontal direction, a vertical direction, or similar to thevertical direction, the reference filter smoothing may be performedusing a first intra filter. On the other hands, when the size of theprediction block relating to the current block is equal to or greaterthan 32×32 and a variation between reference samples is equal to or lessthan a predetermined value (i.e., a change of values of the referencesamples is not large), the reference sample smoothing may be performedusing a second intra filter.

When reference samples included in a reference line is filtered usingthe first intra filter, a filtered reference sample may be derived asshown in Equation 14.P(−1,−1)=(P(−1,0)+2*P(−1,−1)+P(0,−1)+2)>>2P(−1,y)=(P(−1,y+1)+2*P(−1,y)+P(−1,y−1)+2)>>2P(x,−1)=(P(x+1,−1)+2*P(x,−1)+P(x−1,−1)+2)>>2  [Equation 14]

When reference samples included in a reference line is filtered usingthe second intra filter, a filtered reference sample may be derived asshown in Equation 15.P(−1,y)=((2N−y)*P(−1,−1)+(y+1)*P(−1,2N+2K−1)+N/2)>>NP(x,−1)=((2N−x)*P(−1,−1)+(x+1)*P(2N+2K−1,−1)+N/2)>>N  [Equation 15]

In the above equations 14 and 15, x may have a value between 0 and2N−2(k−1), and y may have a value between 0 and 2N−2(k−1). Here, krepresents an index of a reference line, and N represents a size of acurrent block.

Alternatively, a type of an intra filter or the number of times in whichan intra filter is applied may be variably determined according to areference line. For example, different intra filters may be applieddepending on an index or a position of a reference line. Or differentintra filters may be applied depending on a group to which a referenceline belongs. Here, the different filters may be different from eachother in at least one of a length of a filter, a filter coefficient, ora filter strength.

As an example, a plurality of reference lines may be classified into atleast one group. At this time, each group may include at least onereference line. The grouping may be performed based on at least one ofindexes of reference lines, a number of reference lines, or adjacencybetween reference lines. For example, the plurality of reference linesmay be classified into at least one group based on factors such aswhether the index of the reference line is an even number or whether theindex of the reference line is equal to or greater than a predeterminedvalue.

For example, when a current block uses at least one of a first referenceline or a second reference line, intra reference smoothing may beperformed using a first intra filter. On the other hand, when thecurrent block uses at least one of a third reference line or a fourthreference line, intra reference smoothing may be performed using asecond intra filter. Here, the first intra filter and the second intrafilter may be different from each other in at least one of a filtercoefficient, a filter tap or a filter strength. For example, filtercoefficients of the first intra filter may be (1, 2, 1) and filtercoefficients of the second intra filter may be (2, 12, 2).

A filter type to be applied to a reference line may be determined basedon at least one of a size of a current block, a type of an intraprediction mode, a directionality of an intra prediction mode, aposition of the reference line, the number of reference lines. And oneof filters included in the determined filter type may be applied to thereference line based on the enumerated elements. Here, the filter typemay be classified according to at least one of a filter length (numberof taps), a filter coefficient or a filter strength. For example, firsttype filters may have the same filter length as each other, but may havea different filter length from the second type filters.

For example, when a size of a prediction block related to a currentblock is equal to or less than 32×32 and an intra prediction mode of thecurrent block does not have a horizontal direction, a direction similarto the horizontal direction, a vertical direction, or a directionsimilar to the vertical direction, it may be determined to use the firstfilter type. And, when the current block uses at least one of a firstreference line or a second reference line, reference sample smoothingmay be performed using a first intra filter included in the first intrafilter type. On the other hand, when the current block uses at least oneof a third reference line or a fourth reference line, reference samplesmoothing may be performed using a second intra filter included in thefirst intra filter type. At this time, filter lengths of the first intrafilter and the second intra filter are the same while the filtercoefficients are different. For example, the filter coefficient of thefirst intra filter may be (1, 2, 1) and the filter coefficient of thesecond intra filter may be (2, 12, 2).

The number of available intra prediction modes may be variablydetermined depending on whether the extended reference line is used ornot. That is, depending on whether or not the extended reference line isused, a different number of directional intra prediction modes may beused in units of a sequence, slice, a coding tree unit, a coding unit,or a prediction unit.

Since an efficiency of intra prediction is increased when the extendedreference line is used, there is no problem in performing efficientintra prediction even when a smaller number of intra prediction modesare used than when the extended reference line is not used. Thus,depending on whether or not the extended reference line is used, it maybe determined whether to use N or less than N directional predictionmodes. For example, it may be set that base intra prediction modes(i.e., 33 directional intra prediction modes) are used when the extendedreference line is used and extended intra prediction modes (i.e., 65directional intra prediction modes) are used when the extended referenceline is not used.

A type or the number of intra prediction modes available for the currentblock may be limited according to a position of a reference line usedfor intra prediction.

For example, as a distance between a current block and a reference lineincreases, discontinuity between the current block and the referenceline increases. Accordingly, as the distance between the current blockand the reference line increases, efficiency of intra prediction using anon-directional prediction mode such as DC mode or planar modedecreases. Accordingly, it may be set not to use non-directionalprediction mode including at least one of DC mode or planar mode when areference line having a distance from the current block equal to orgreater than a predetermined threshold value is used. For example, whenan index of the reference line used for intra prediction of the currentblock is equal to or greater than L, the non-directional prediction modeincluding at least one of DC mode and planar mode may not be used. Here,L is an integer including 0, and may be 0, 1, 2, 3, and so on.

Alternatively, the number of available intra prediction modes may beadjusted according to a position of an intra reference line used forintra prediction of a current block. For example, when an index of areference line used for intra prediction of the current block is equalto or greater than L, base intra prediction modes may be used, and whenan index of a reference line is smaller than L, extended intraprediction modes may be used. For example, if L is 2, the extended intraprediction modes (i.e., 67 intra prediction modes) may be used for thefirst reference line or the second reference line, and the base intraprediction modes (i.e., 35 intra prediction modes) may be used for thethird reference line or the fourth reference line.

Depending on an intra prediction mode of a current block or the numberof intra prediction modes available for the current block, a referenceline that is available to be selected for the current block may belimited. For example, when the intra prediction mode of the currentblock is a non-directional prediction mode such as DC mode or planarmode, it may be set that a reference line having an index equal to orgreater than L is not used for intra prediction of the current block.

Above embodiments have been described mainly on decoding process,encoding process may be performed in the same order as described or inreverse order.

FIG. 27 is a flowchart illustrating processes of obtaining a residualsample according to an embodiment to which the present invention isapplied.

First, a residual coefficient of a current block may be obtained S2710.A decoder may obtain a residual coefficient through a coefficientscanning method. For example, the decoder may perform a coefficient scanusing a jig-zag scan, a vertical scan, or a horizontal scan, and mayobtain residual coefficients in a form of a two-dimensional block.

An inverse quantization may be performed on the residual coefficient ofthe current block S2720.

It is possible to determine whether to skip an inverse transform on thedequantized residual coefficient of the current block S2730.Specifically, the decoder may determine whether to skip the inversetransform on at least one of a horizontal direction or a verticaldirection of the current block. When it is determined to apply theinverse transform on at least one of the horizontal direction or thevertical direction of the current block, a residual sample of thecurrent block may be obtained by inverse transforming the dequantizedresidual coefficient of the current block. Here, the inverse transformcan be performed using at least one of DCT, DST, and KLT.

When the inverse transform is skipped in both the horizontal directionand the vertical direction of the current block, inverse transform isnot performed in the horizontal direction and the vertical direction ofthe current block. In this case, the residual sample of the currentblock may be obtained by scaling the dequantized residual coefficientwith a predetermined value.

Skipping the inverse transform on the horizontal direction means thatthe inverse transform is not performed on the horizontal direction butthe inverse transform is performed on the vertical direction. At thistime, scaling may be performed in the horizontal direction.

Skipping the inverse transform on the vertical direction means that theinverse transform is not performed on the vertical direction but theinverse transform is performed on the horizontal direction. At thistime, scaling may be performed in the vertical direction.

It may be determined whether or not an inverse transform skip techniquemay be used for the current block depending on a partition type of thecurrent block. For example, if the current block is generated through abinary tree-based partitioning, the inverse transform skip scheme may berestricted for the current block. Accordingly, when the current block isgenerated through the binary tree-based partitioning, the residualsample of the current block may be obtained by inverse transforming thecurrent block. In addition, when the current block is generated throughbinary tree-based partitioning, encoding/decoding of informationindicating whether or not the inverse transform is skipped (e.g.,transform_skip_flag) may be omitted.

Alternatively, when the current block is generated through binarytree-based partitioning, it is possible to limit the inverse transformskip scheme to at least one of the horizontal direction or the verticaldirection. Here, the direction in which the inverse transform skipscheme is limited may be determined based on information decoded fromthe bitstream, or may be adaptively determined based on at least one ofa size of the current block, a shape of the current block, or an intraprediction mode of the current block.

For example, when the current block is a non-square block having a widthgreater than a height, the inverse transform skip scheme may be allowedonly in the vertical direction and restricted in the horizontaldirection. That is, when the current block is 2N×N, the inversetransform is performed in the horizontal direction of the current block,and the inverse transform may be selectively performed in the verticaldirection.

On the other hand, when the current block is a non-square block having aheight greater than a width, the inverse transform skip scheme may beallowed only in the horizontal direction and restricted in the verticaldirection. That is, when the current block is N×2N, the inversetransform is performed in the vertical direction of the current block,and the inverse transform may be selectively performed in the horizontaldirection.

In contrast to the above example, when the current block is a non-squareblock having a width greater than a height, the inverse transform skipscheme may be allowed only in the horizontal direction, and when thecurrent block is a non-square block having a height greater than awidth, the inverse transform skip scheme may be allowed only in thevertical direction.

Information indicating whether or not to skip the inverse transform withrespect to the horizontal direction or information indicating whether toskip the inverse transformation with respect to the vertical directionmay be signaled through a bitstream. For example, the informationindicating whether or not to skip the inverse transform on thehorizontal direction is a 1-bit flag, ‘hor_transform_skip_flag’, andinformation indicating whether to skip the inverse transform on thevertical direction is a 1-bit flag, ‘ver_transform_skip_flag_’. Theencoder may encode at least one of ‘hor_transform_skip_flag’ or‘ver_transform skip_flag’ according to the shape of the current block.Further, the decoder may determine whether or not the inverse transformon the horizontal direction or on the vertical direction is skipped byusing at least one of “hor_transform_skip_flag” or“ver_transform_skip_flag”.

It may be set to skip the inverse transform for any one direction of thecurrent block depending on a partition type of the current block. Forexample, if the current block is generated through a binary tree-basedpartitioning, the inverse transform on the horizontal direction orvertical direction may be skipped. That is, if the current block isgenerated by binary tree-based partitioning, it may be determined thatthe inverse transform for the current block is skipped on at least oneof a horizontal direction or a vertical direction withoutencoding/decoding information (e.g., transform_skip flag,hor_transform_skip_flag, ver_transform_skip_flag) indicating whether ornot the inverse transform of the current block is skipped.

Although the above-described embodiments have been described on thebasis of a series of steps or flowcharts, they do not limit thetime-series order of the invention, and may be performed simultaneouslyor in different orders as necessary. Further, each of the components(for example, units, modules, etc.) constituting the block diagram inthe above-described embodiments may be implemented by a hardware deviceor software, and a plurality of components. Or a plurality of componentsmay be combined and implemented by a single hardware device or software.The above-described embodiments may be implemented in the form ofprogram instructions that may be executed through various computercomponents and recorded in a computer-readable recording medium. Thecomputer-readable recording medium may include one of or combination ofprogram commands, data files, data structures, and the like. Examples ofcomputer-readable media include magnetic media such as hard disks,floppy disks and magnetic tape, optical recording media such as CD-ROMsand DVDs, magneto-optical media such as floptical disks, media, andhardware devices specifically configured to store and execute programinstructions such as ROM, RAM, flash memory, and the like. The hardwaredevice may be configured to operate as one or more software modules forperforming the process according to the present invention, and viceversa.

INDUSTRIAL APPLICABILITY

The present invention may be applied to electronic devices which is ableto encode/decode a video.

The invention claimed is:
 1. A method for decoding a video, the methodcomprising: selecting a reference sample line of a current block among aplurality of reference sample lines; performing intra prediction for thecurrent block based on the selected reference sample line and an intraprediction mode of the current block; and reconstructing the currentblock based on prediction samples of the current block, wherein if adistance between the current block and the selected reference sampleline is greater than a pre-determined value, at least onenon-directional intra prediction mode is unavailable as the intraprediction mode of the current block, the at least one non-directionalintra prediction mode comprising at least one of DC mode or Planar mode.2. The method of claim 1, wherein when the intra prediction mode of thecurrent block has a direction of top-right diagonal or bottom-leftdiagonal, a prediction sample resultant from the intra prediction ismodified by using a reference sample included in a neighboring blockadjacent to the current block.
 3. The method of claim 2, wherein thereference sample comprises at least one of a sample toward a reversedirection of the intra prediction mode.
 4. A method for encoding avideo, the method comprising: selecting a reference sample line of acurrent block among a plurality of reference sample lines; performingintra prediction for the current block based on the selected referencesample line and an intra prediction mode of the current block; andobtaining residual samples by subtracting prediction samples fromoriginal samples, wherein if a distance between the current block andthe selected reference sample line is greater than a pre-determinedvalue, at least one non-directional intra prediction mode is unavailableas the intra prediction mode of the current block, the at least onenon-directional intra prediction mode comprising at least one of DC modeor Planar mode.
 5. The method of claim 4, wherein when the intraprediction mode of the current block has a direction of top-rightdiagonal or bottom-left diagonal, a prediction sample resultant from theintra prediction is modified by using a reference sample included in aneighboring block adjacent to the current block.
 6. The method of claim5, wherein the reference sample comprises at least one of a sampletoward a reverse direction of the intra prediction mode.
 7. Anon-transitory computer-readable medium for storing compressed dataassociated with a video signal, the compressed data comprising: indexinformation for: selecting a reference sample line of a current blockamong a plurality of reference sample lines, wherein the current blockis reconstructed based on prediction samples resulting from intraprediction for the current block, wherein the intra prediction for thecurrent block is performed based on the reference sample line selectedby the index information and an intra prediction mode of the currentblock; wherein if a distance between the current block and the selectedreference sample line is greater than a pre-determined value, at leastone non-directional intra prediction mode is unavailable as the intraprediction mode of the current block, and wherein the at least onenon-directional intra prediction mode comprises at least one of DC modeor Planar mode.